chore: make BrowserObserver a CheckedObserver

refs: 1053338

atom/browser/native_window_observer.h

@@ -7,6 +7,7 @@
 
 #include <string>
 
+#include "base/observer_list_types.h"
 #include "base/strings/string16.h"
 #include "base/values.h"
 #include "ui/base/window_open_disposition.h"
@@ -22,9 +23,9 @@ class Rect;
 
 namespace atom {
 
-class NativeWindowObserver {
+class NativeWindowObserver : public base::CheckedObserver {
  public:
-  virtual ~NativeWindowObserver() {}
+  ~NativeWindowObserver() override {}
 
   // Called when the web page in window wants to create a popup window.
   virtual void WillCreatePopupWindow(const base::string16& frame_name,