diff --git a/shell/browser/api/electron_api_app.h b/shell/browser/api/electron_api_app.h
index 96f47b273a1c..f03fc96d855f 100644
--- a/shell/browser/api/electron_api_app.h
+++ b/shell/browser/api/electron_api_app.h
@@ -50,9 +50,9 @@ namespace api {
 class App : public ElectronBrowserClient::Delegate,
             public gin::Wrappable<App>,
             public gin_helper::EventEmitterMixin<App>,
-            public BrowserObserver,
-            public content::GpuDataManagerObserver,
-            public content::BrowserChildProcessObserver {
+            private BrowserObserver,
+            private content::GpuDataManagerObserver,
+            private content::BrowserChildProcessObserver {
  public:
   using FileIconCallback =
       base::RepeatingCallback<void(v8::Local<v8::Value>, const gfx::Image&)>;
diff --git a/shell/browser/api/electron_api_auto_updater.h b/shell/browser/api/electron_api_auto_updater.h
index aadbd7543c26..50c771fddbb0 100644
--- a/shell/browser/api/electron_api_auto_updater.h
+++ b/shell/browser/api/electron_api_auto_updater.h
@@ -18,7 +18,7 @@ namespace electron::api {
 class AutoUpdater : public gin::Wrappable<AutoUpdater>,
                     public gin_helper::EventEmitterMixin<AutoUpdater>,
                     public auto_updater::Delegate,
-                    public WindowListObserver {
+                    private WindowListObserver {
  public:
   static gin::Handle<AutoUpdater> Create(v8::Isolate* isolate);
 
diff --git a/shell/browser/api/electron_api_base_window.h b/shell/browser/api/electron_api_base_window.h
index 46043f284f5e..c4bd0a879919 100644
--- a/shell/browser/api/electron_api_base_window.h
+++ b/shell/browser/api/electron_api_base_window.h
@@ -26,7 +26,7 @@ namespace electron::api {
 class View;
 
 class BaseWindow : public gin_helper::TrackableObject<BaseWindow>,
-                   public NativeWindowObserver {
+                   private NativeWindowObserver {
  public:
   static gin_helper::WrappableBase* New(gin_helper::Arguments* args);
 
diff --git a/shell/browser/api/electron_api_browser_window.h b/shell/browser/api/electron_api_browser_window.h
index e752d1ed595d..2ad029bc0351 100644
--- a/shell/browser/api/electron_api_browser_window.h
+++ b/shell/browser/api/electron_api_browser_window.h
@@ -16,8 +16,8 @@
 namespace electron::api {
 
 class BrowserWindow : public BaseWindow,
-                      public content::WebContentsObserver,
-                      public ExtendedWebContentsObserver {
+                      private content::WebContentsObserver,
+                      private ExtendedWebContentsObserver {
  public:
   static gin_helper::WrappableBase* New(gin_helper::ErrorThrower thrower,
                                         gin::Arguments* args);
diff --git a/shell/browser/api/electron_api_debugger.h b/shell/browser/api/electron_api_debugger.h
index 74299619487f..deda78c6cd28 100644
--- a/shell/browser/api/electron_api_debugger.h
+++ b/shell/browser/api/electron_api_debugger.h
@@ -28,7 +28,7 @@ namespace electron::api {
 class Debugger : public gin::Wrappable<Debugger>,
                  public gin_helper::EventEmitterMixin<Debugger>,
                  public content::DevToolsAgentHostClient,
-                 public content::WebContentsObserver {
+                 private content::WebContentsObserver {
  public:
   static gin::Handle<Debugger> Create(v8::Isolate* isolate,
                                       content::WebContents* web_contents);
diff --git a/shell/browser/api/electron_api_download_item.h b/shell/browser/api/electron_api_download_item.h
index 7b240aa10ad8..f04953a8d3dc 100644
--- a/shell/browser/api/electron_api_download_item.h
+++ b/shell/browser/api/electron_api_download_item.h
@@ -24,7 +24,7 @@ namespace electron::api {
 class DownloadItem : public gin::Wrappable<DownloadItem>,
                      public gin_helper::Pinnable<DownloadItem>,
                      public gin_helper::EventEmitterMixin<DownloadItem>,
-                     public download::DownloadItem::Observer {
+                     private download::DownloadItem::Observer {
  public:
   static gin::Handle<DownloadItem> FromOrCreate(v8::Isolate* isolate,
                                                 download::DownloadItem* item);
diff --git a/shell/browser/api/electron_api_in_app_purchase.h b/shell/browser/api/electron_api_in_app_purchase.h
index 4b646410545d..e89a7ca691ba 100644
--- a/shell/browser/api/electron_api_in_app_purchase.h
+++ b/shell/browser/api/electron_api_in_app_purchase.h
@@ -20,7 +20,7 @@ namespace electron::api {
 
 class InAppPurchase : public gin::Wrappable<InAppPurchase>,
                       public gin_helper::EventEmitterMixin<InAppPurchase>,
-                      public in_app_purchase::TransactionObserver {
+                      private in_app_purchase::TransactionObserver {
  public:
   static gin::Handle<InAppPurchase> Create(v8::Isolate* isolate);
 
diff --git a/shell/browser/api/electron_api_menu.h b/shell/browser/api/electron_api_menu.h
index 697fbd950fc0..c12fda7e9353 100644
--- a/shell/browser/api/electron_api_menu.h
+++ b/shell/browser/api/electron_api_menu.h
@@ -24,7 +24,7 @@ class Menu : public gin::Wrappable<Menu>,
              public gin_helper::Constructible<Menu>,
              public gin_helper::Pinnable<Menu>,
              public ElectronMenuModel::Delegate,
-             public ElectronMenuModel::Observer {
+             private ElectronMenuModel::Observer {
  public:
   // gin_helper::Constructible
   static gin::Handle<Menu> New(gin::Arguments* args);
diff --git a/shell/browser/api/electron_api_native_theme.h b/shell/browser/api/electron_api_native_theme.h
index 6be9c008e03b..a1f7798014b6 100644
--- a/shell/browser/api/electron_api_native_theme.h
+++ b/shell/browser/api/electron_api_native_theme.h
@@ -16,7 +16,7 @@ namespace electron::api {
 
 class NativeTheme : public gin::Wrappable<NativeTheme>,
                     public gin_helper::EventEmitterMixin<NativeTheme>,
-                    public ui::NativeThemeObserver {
+                    private ui::NativeThemeObserver {
  public:
   static gin::Handle<NativeTheme> Create(v8::Isolate* isolate);
 
diff --git a/shell/browser/api/electron_api_power_monitor.h b/shell/browser/api/electron_api_power_monitor.h
index 01f3aced32f4..436fd9c0f87f 100644
--- a/shell/browser/api/electron_api_power_monitor.h
+++ b/shell/browser/api/electron_api_power_monitor.h
@@ -20,9 +20,9 @@ namespace electron::api {
 class PowerMonitor : public gin::Wrappable<PowerMonitor>,
                      public gin_helper::EventEmitterMixin<PowerMonitor>,
                      public gin_helper::Pinnable<PowerMonitor>,
-                     public base::PowerStateObserver,
-                     public base::PowerSuspendObserver,
-                     public base::PowerThermalObserver {
+                     private base::PowerStateObserver,
+                     private base::PowerSuspendObserver,
+                     private base::PowerThermalObserver {
  public:
   static v8::Local<v8::Value> Create(v8::Isolate* isolate);
 
diff --git a/shell/browser/api/electron_api_push_notifications.h b/shell/browser/api/electron_api_push_notifications.h
index 19f29d83d5dd..6f1e9f1215b7 100644
--- a/shell/browser/api/electron_api_push_notifications.h
+++ b/shell/browser/api/electron_api_push_notifications.h
@@ -21,7 +21,7 @@ class PushNotifications
     : public ElectronBrowserClient::Delegate,
       public gin::Wrappable<PushNotifications>,
       public gin_helper::EventEmitterMixin<PushNotifications>,
-      public BrowserObserver {
+      private BrowserObserver {
  public:
   static PushNotifications* Get();
   static gin::Handle<PushNotifications> Create(v8::Isolate* isolate);
diff --git a/shell/browser/api/electron_api_screen.h b/shell/browser/api/electron_api_screen.h
index ca29a79f5d06..19ce4d40dbc6 100644
--- a/shell/browser/api/electron_api_screen.h
+++ b/shell/browser/api/electron_api_screen.h
@@ -24,7 +24,7 @@ namespace electron::api {
 
 class Screen : public gin::Wrappable<Screen>,
                public gin_helper::EventEmitterMixin<Screen>,
-               public display::DisplayObserver {
+               private display::DisplayObserver {
  public:
   static v8::Local<v8::Value> Create(gin_helper::ErrorThrower error_thrower);
 
diff --git a/shell/browser/api/electron_api_service_worker_context.h b/shell/browser/api/electron_api_service_worker_context.h
index 9a6a6cd67162..52f617e59e0c 100644
--- a/shell/browser/api/electron_api_service_worker_context.h
+++ b/shell/browser/api/electron_api_service_worker_context.h
@@ -21,7 +21,7 @@ namespace api {
 class ServiceWorkerContext
     : public gin::Wrappable<ServiceWorkerContext>,
       public gin_helper::EventEmitterMixin<ServiceWorkerContext>,
-      public content::ServiceWorkerContextObserver {
+      private content::ServiceWorkerContextObserver {
  public:
   static gin::Handle<ServiceWorkerContext> Create(
       v8::Isolate* isolate,
diff --git a/shell/browser/api/electron_api_session.cc b/shell/browser/api/electron_api_session.cc
index 69e10b10f71b..191f632e7516 100644
--- a/shell/browser/api/electron_api_session.cc
+++ b/shell/browser/api/electron_api_session.cc
@@ -262,7 +262,7 @@ class ClearDataTask {
   // of a full |ClearDataTask|. This class manages its own lifetime, cleaning
   // itself up after the operation completes and notifies the task of the
   // result.
-  class ClearDataOperation : public BrowsingDataRemover::Observer {
+  class ClearDataOperation : private BrowsingDataRemover::Observer {
    public:
     static void Run(std::shared_ptr<ClearDataTask> task,
                     BrowsingDataRemover* remover,
diff --git a/shell/browser/api/electron_api_session.h b/shell/browser/api/electron_api_session.h
index 1efe2296fa2e..0af8a72f0c9f 100644
--- a/shell/browser/api/electron_api_session.h
+++ b/shell/browser/api/electron_api_session.h
@@ -65,12 +65,12 @@ class Session : public gin::Wrappable<Session>,
                 public gin_helper::EventEmitterMixin<Session>,
                 public gin_helper::CleanedUpAtExit,
 #if BUILDFLAG(ENABLE_BUILTIN_SPELLCHECKER)
-                public SpellcheckHunspellDictionary::Observer,
+                private SpellcheckHunspellDictionary::Observer,
 #endif
 #if BUILDFLAG(ENABLE_ELECTRON_EXTENSIONS)
-                public extensions::ExtensionRegistryObserver,
+                private extensions::ExtensionRegistryObserver,
 #endif
-                public content::DownloadManager::Observer {
+                private content::DownloadManager::Observer {
  public:
   // Gets or creates Session from the |browser_context|.
   static gin::Handle<Session> CreateFrom(
diff --git a/shell/browser/api/electron_api_tray.h b/shell/browser/api/electron_api_tray.h
index 26429e9ee823..9eaebb506663 100644
--- a/shell/browser/api/electron_api_tray.h
+++ b/shell/browser/api/electron_api_tray.h
@@ -39,7 +39,7 @@ class Tray : public gin::Wrappable<Tray>,
              public gin_helper::Constructible<Tray>,
              public gin_helper::CleanedUpAtExit,
              public gin_helper::Pinnable<Tray>,
-             public TrayIconObserver {
+             private TrayIconObserver {
  public:
   // gin_helper::Constructible
   static gin::Handle<Tray> New(gin_helper::ErrorThrower thrower,
diff --git a/shell/browser/api/electron_api_web_contents.h b/shell/browser/api/electron_api_web_contents.h
index a34a4bbe805c..07fd3ad8daa8 100644
--- a/shell/browser/api/electron_api_web_contents.h
+++ b/shell/browser/api/electron_api_web_contents.h
@@ -111,7 +111,7 @@ class WebContents : public ExclusiveAccessContext,
                     public gin_helper::CleanedUpAtExit,
                     public content::WebContentsObserver,
                     public content::WebContentsDelegate,
-                    public content::RenderWidgetHost::InputEventObserver,
+                    private content::RenderWidgetHost::InputEventObserver,
                     public content::JavaScriptDialogManager,
                     public InspectableWebContentsDelegate,
                     public InspectableWebContentsViewDelegate,
diff --git a/shell/browser/api/electron_api_web_contents_view.h b/shell/browser/api/electron_api_web_contents_view.h
index 353086427ba6..b02c3a460d58 100644
--- a/shell/browser/api/electron_api_web_contents_view.h
+++ b/shell/browser/api/electron_api_web_contents_view.h
@@ -21,7 +21,7 @@ namespace electron::api {
 class WebContents;
 
 class WebContentsView : public View,
-                        public content::WebContentsObserver,
+                        private content::WebContentsObserver,
                         public DraggableRegionProvider {
  public:
   // Create a new instance of WebContentsView.
diff --git a/shell/browser/electron_api_ipc_handler_impl.h b/shell/browser/electron_api_ipc_handler_impl.h
index 1a0ad3a0135d..96e9a620ebd5 100644
--- a/shell/browser/electron_api_ipc_handler_impl.h
+++ b/shell/browser/electron_api_ipc_handler_impl.h
@@ -20,7 +20,7 @@ class RenderFrameHost;
 
 namespace electron {
 class ElectronApiIPCHandlerImpl : public mojom::ElectronApiIPC,
-                                  public content::WebContentsObserver {
+                                  private content::WebContentsObserver {
  public:
   explicit ElectronApiIPCHandlerImpl(
       content::RenderFrameHost* render_frame_host,
diff --git a/shell/browser/electron_browser_client.h b/shell/browser/electron_browser_client.h
index 9f3f7fe8dfdf..39e6b90c7228 100644
--- a/shell/browser/electron_browser_client.h
+++ b/shell/browser/electron_browser_client.h
@@ -43,7 +43,7 @@ class PlatformNotificationService;
 class ElectronWebAuthenticationDelegate;
 
 class ElectronBrowserClient : public content::ContentBrowserClient,
-                              public content::RenderProcessHostObserver {
+                              private content::RenderProcessHostObserver {
  public:
   static ElectronBrowserClient* Get();
   static void SetApplicationLocale(const std::string& locale);
diff --git a/shell/browser/electron_web_contents_utility_handler_impl.h b/shell/browser/electron_web_contents_utility_handler_impl.h
index 79bca6b664fe..2f0f07d007ed 100644
--- a/shell/browser/electron_web_contents_utility_handler_impl.h
+++ b/shell/browser/electron_web_contents_utility_handler_impl.h
@@ -21,7 +21,7 @@ class RenderFrameHost;
 namespace electron {
 class ElectronWebContentsUtilityHandlerImpl
     : public mojom::ElectronWebContentsUtility,
-      public content::WebContentsObserver {
+      private content::WebContentsObserver {
  public:
   explicit ElectronWebContentsUtilityHandlerImpl(
       content::RenderFrameHost* render_frame_host,
diff --git a/shell/browser/file_select_helper.h b/shell/browser/file_select_helper.h
index 27a2dbb0b20a..7ade0b04d394 100644
--- a/shell/browser/file_select_helper.h
+++ b/shell/browser/file_select_helper.h
@@ -41,7 +41,7 @@ class FileSelectHelper : public base::RefCountedThreadSafe<
                              FileSelectHelper,
                              content::BrowserThread::DeleteOnUIThread>,
                          public ui::SelectFileDialog::Listener,
-                         public content::WebContentsObserver,
+                         private content::WebContentsObserver,
                          private net::DirectoryLister::DirectoryListerDelegate {
  public:
   // disable copy
diff --git a/shell/browser/hid/electron_hid_delegate.cc b/shell/browser/hid/electron_hid_delegate.cc
index 10aa32c41dcf..91a174ab2c5f 100644
--- a/shell/browser/hid/electron_hid_delegate.cc
+++ b/shell/browser/hid/electron_hid_delegate.cc
@@ -41,7 +41,7 @@ namespace electron {
 
 // Manages the HidDelegate observers for a single browser context.
 class ElectronHidDelegate::ContextObservation
-    : public HidChooserContext::DeviceObserver {
+    : private HidChooserContext::DeviceObserver {
  public:
   ContextObservation(ElectronHidDelegate* parent,
                      content::BrowserContext* browser_context)
diff --git a/shell/browser/login_handler.h b/shell/browser/login_handler.h
index 0e513f8c98b2..1f229baaa37d 100644
--- a/shell/browser/login_handler.h
+++ b/shell/browser/login_handler.h
@@ -22,7 +22,7 @@ namespace electron {
 
 // Handles HTTP basic auth.
 class LoginHandler : public content::LoginDelegate,
-                     public content::WebContentsObserver {
+                     private content::WebContentsObserver {
  public:
   LoginHandler(const net::AuthChallengeInfo& auth_info,
                content::WebContents* web_contents,
diff --git a/shell/browser/native_window_views.h b/shell/browser/native_window_views.h
index e5683cecf00d..81c39adfc003 100644
--- a/shell/browser/native_window_views.h
+++ b/shell/browser/native_window_views.h
@@ -38,8 +38,8 @@ gfx::Rect ScreenToDIPRect(HWND hwnd, const gfx::Rect& pixel_bounds);
 #endif
 
 class NativeWindowViews : public NativeWindow,
-                          public views::WidgetObserver,
-                          public ui::EventHandler {
+                          private views::WidgetObserver,
+                          private ui::EventHandler {
  public:
   NativeWindowViews(const gin_helper::Dictionary& options,
                     NativeWindow* parent);
diff --git a/shell/browser/osr/osr_render_widget_host_view.h b/shell/browser/osr/osr_render_widget_host_view.h
index b674f7cfbcf6..2ce5204d9751 100644
--- a/shell/browser/osr/osr_render_widget_host_view.h
+++ b/shell/browser/osr/osr_render_widget_host_view.h
@@ -61,9 +61,9 @@ typedef base::RepeatingCallback<void(const gfx::Rect&)> OnPopupPaintCallback;
 
 class OffScreenRenderWidgetHostView
     : public content::RenderWidgetHostViewBase,
-      public content::RenderFrameMetadataProvider::Observer,
+      private content::RenderFrameMetadataProvider::Observer,
       public ui::CompositorDelegate,
-      public OffscreenViewProxyObserver {
+      private OffscreenViewProxyObserver {
  public:
   OffScreenRenderWidgetHostView(bool transparent,
                                 bool painting,
diff --git a/shell/browser/osr/osr_web_contents_view.h b/shell/browser/osr/osr_web_contents_view.h
index 18587533ba59..89bf556c0820 100644
--- a/shell/browser/osr/osr_web_contents_view.h
+++ b/shell/browser/osr/osr_web_contents_view.h
@@ -28,7 +28,7 @@ namespace electron {
 
 class OffScreenWebContentsView : public content::WebContentsView,
                                  public content::RenderViewHostDelegateView,
-                                 public NativeWindowObserver {
+                                 private NativeWindowObserver {
  public:
   OffScreenWebContentsView(bool transparent, const OnPaintCallback& callback);
   ~OffScreenWebContentsView() override;
diff --git a/shell/browser/serial/electron_serial_delegate.h b/shell/browser/serial/electron_serial_delegate.h
index dd12e4484eb3..986fa28a5709 100644
--- a/shell/browser/serial/electron_serial_delegate.h
+++ b/shell/browser/serial/electron_serial_delegate.h
@@ -20,7 +20,7 @@ namespace electron {
 class SerialChooserController;
 
 class ElectronSerialDelegate : public content::SerialDelegate,
-                               public SerialChooserContext::PortObserver {
+                               private SerialChooserContext::PortObserver {
  public:
   ElectronSerialDelegate();
   ~ElectronSerialDelegate() override;
diff --git a/shell/browser/ui/electron_desktop_window_tree_host_linux.h b/shell/browser/ui/electron_desktop_window_tree_host_linux.h
index 27f81d4a9998..29bc4254a4b3 100644
--- a/shell/browser/ui/electron_desktop_window_tree_host_linux.h
+++ b/shell/browser/ui/electron_desktop_window_tree_host_linux.h
@@ -23,8 +23,8 @@ namespace electron {
 
 class ElectronDesktopWindowTreeHostLinux
     : public views::DesktopWindowTreeHostLinux,
-      public ui::NativeThemeObserver,
-      public ui::DeviceScaleFactorObserver {
+      private ui::NativeThemeObserver,
+      private ui::DeviceScaleFactorObserver {
  public:
   ElectronDesktopWindowTreeHostLinux(
       NativeWindowViews* native_window_view,
diff --git a/shell/browser/ui/inspectable_web_contents.h b/shell/browser/ui/inspectable_web_contents.h
index fb4d2adbe4cc..89d0d965ff58 100644
--- a/shell/browser/ui/inspectable_web_contents.h
+++ b/shell/browser/ui/inspectable_web_contents.h
@@ -34,7 +34,7 @@ class InspectableWebContentsView;
 
 class InspectableWebContents
     : public content::DevToolsAgentHostClient,
-      public content::WebContentsObserver,
+      private content::WebContentsObserver,
       public content::WebContentsDelegate,
       public DevToolsEmbedderMessageDispatcher::Delegate {
  public:
diff --git a/shell/browser/ui/message_box_gtk.cc b/shell/browser/ui/message_box_gtk.cc
index 51bb743b0d46..c882be2aaa0c 100644
--- a/shell/browser/ui/message_box_gtk.cc
+++ b/shell/browser/ui/message_box_gtk.cc
@@ -47,7 +47,7 @@ base::flat_map<int, GtkWidget*>& GetDialogsMap() {
   return *dialogs;
 }
 
-class GtkMessageBox : public NativeWindowObserver {
+class GtkMessageBox : private NativeWindowObserver {
  public:
   explicit GtkMessageBox(const MessageBoxSettings& settings)
       : id_(settings.id),
diff --git a/shell/browser/ui/views/autofill_popup_view.h b/shell/browser/ui/views/autofill_popup_view.h
index 88420bb4cd74..cabc1182d567 100644
--- a/shell/browser/ui/views/autofill_popup_view.h
+++ b/shell/browser/ui/views/autofill_popup_view.h
@@ -58,8 +58,8 @@ class AutofillPopupChildView : public views::View {
 };
 
 class AutofillPopupView : public views::WidgetDelegateView,
-                          public views::WidgetFocusChangeListener,
-                          public views::WidgetObserver,
+                          private views::WidgetFocusChangeListener,
+                          private views::WidgetObserver,
                           public views::DragController {
  public:
   explicit AutofillPopupView(AutofillPopup* popup,
diff --git a/shell/browser/ui/views/client_frame_view_linux.h b/shell/browser/ui/views/client_frame_view_linux.h
index ce60e43c575a..05d5b5454f0c 100644
--- a/shell/browser/ui/views/client_frame_view_linux.h
+++ b/shell/browser/ui/views/client_frame_view_linux.h
@@ -29,8 +29,8 @@
 namespace electron {
 
 class ClientFrameViewLinux : public FramelessView,
-                             public ui::NativeThemeObserver,
-                             public ui::WindowButtonOrderObserver {
+                             private ui::NativeThemeObserver,
+                             private ui::WindowButtonOrderObserver {
   METADATA_HEADER(ClientFrameViewLinux, FramelessView)
 
  public:
diff --git a/shell/browser/ui/views/menu_bar.h b/shell/browser/ui/views/menu_bar.h
index 0db03ae310a6..b2e1860ea2f0 100644
--- a/shell/browser/ui/views/menu_bar.h
+++ b/shell/browser/ui/views/menu_bar.h
@@ -21,8 +21,8 @@ class MenuButton;
 namespace electron {
 
 class MenuBar : public views::AccessiblePaneView,
-                public MenuDelegate::Observer,
-                public NativeWindowObserver {
+                private MenuDelegate::Observer,
+                private NativeWindowObserver {
   METADATA_HEADER(MenuBar, views::AccessiblePaneView)
 
  public:
diff --git a/shell/browser/ui/views/win_caption_button_container.h b/shell/browser/ui/views/win_caption_button_container.h
index f4568016af8e..308f35ca5375 100644
--- a/shell/browser/ui/views/win_caption_button_container.h
+++ b/shell/browser/ui/views/win_caption_button_container.h
@@ -27,7 +27,7 @@ class WinCaptionButton;
 // frame and browser window as needed. When extended horizontally, becomes a
 // grab bar for moving the window.
 class WinCaptionButtonContainer : public views::View,
-                                  public views::WidgetObserver {
+                                  private views::WidgetObserver {
   METADATA_HEADER(WinCaptionButtonContainer, views::View)
 
  public:
diff --git a/shell/browser/usb/electron_usb_delegate.cc b/shell/browser/usb/electron_usb_delegate.cc
index a90896e0fe1f..0e2e5c549ca5 100644
--- a/shell/browser/usb/electron_usb_delegate.cc
+++ b/shell/browser/usb/electron_usb_delegate.cc
@@ -88,7 +88,7 @@ namespace electron {
 
 // Manages the UsbDelegate observers for a single browser context.
 class ElectronUsbDelegate::ContextObservation
-    : public UsbChooserContext::DeviceObserver {
+    : private UsbChooserContext::DeviceObserver {
  public:
   ContextObservation(ElectronUsbDelegate* parent,
                      content::BrowserContext* browser_context)
diff --git a/shell/common/api/electron_api_url_loader.h b/shell/common/api/electron_api_url_loader.h
index b0da4a727ba3..8b9f1f913401 100644
--- a/shell/common/api/electron_api_url_loader.h
+++ b/shell/common/api/electron_api_url_loader.h
@@ -46,8 +46,8 @@ namespace electron::api {
 class SimpleURLLoaderWrapper
     : public gin::Wrappable<SimpleURLLoaderWrapper>,
       public gin_helper::EventEmitterMixin<SimpleURLLoaderWrapper>,
-      public network::SimpleURLLoaderStreamConsumer,
-      public network::mojom::URLLoaderNetworkServiceObserver {
+      private network::SimpleURLLoaderStreamConsumer,
+      private network::mojom::URLLoaderNetworkServiceObserver {
  public:
   ~SimpleURLLoaderWrapper() override;
   static gin::Handle<SimpleURLLoaderWrapper> Create(gin::Arguments* args);
diff --git a/shell/renderer/api/electron_api_ipc_renderer.cc b/shell/renderer/api/electron_api_ipc_renderer.cc
index 317b93368749..7eceed45bd9f 100644
--- a/shell/renderer/api/electron_api_ipc_renderer.cc
+++ b/shell/renderer/api/electron_api_ipc_renderer.cc
@@ -42,7 +42,7 @@ RenderFrame* GetCurrentRenderFrame() {
 }
 
 class IPCRenderer : public gin::Wrappable<IPCRenderer>,
-                    public content::RenderFrameObserver {
+                    private content::RenderFrameObserver {
  public:
   static gin::WrapperInfo kWrapperInfo;
 
diff --git a/shell/renderer/api/electron_api_web_frame.cc b/shell/renderer/api/electron_api_web_frame.cc
index ceb5608ab7dd..64dd277ae80a 100644
--- a/shell/renderer/api/electron_api_web_frame.cc
+++ b/shell/renderer/api/electron_api_web_frame.cc
@@ -272,7 +272,7 @@ class FrameSetSpellChecker : public content::RenderFrameVisitor {
   content::RenderFrame* main_frame_;
 };
 
-class SpellCheckerHolder final : public content::RenderFrameObserver {
+class SpellCheckerHolder final : private content::RenderFrameObserver {
  public:
   // Find existing holder for the |render_frame|.
   static SpellCheckerHolder* FromRenderFrame(
@@ -330,7 +330,7 @@ class SpellCheckerHolder final : public content::RenderFrameObserver {
 }  // namespace
 
 class WebFrameRenderer : public gin::Wrappable<WebFrameRenderer>,
-                         public content::RenderFrameObserver {
+                         private content::RenderFrameObserver {
  public:
   static gin::WrapperInfo kWrapperInfo;