diff --git a/atom/browser/native_window.cc b/atom/browser/native_window.cc
index f8023222fb79..641bdd3757a1 100644
--- a/atom/browser/native_window.cc
+++ b/atom/browser/native_window.cc
@@ -170,14 +170,6 @@ void NativeWindow::InitFromOptions(const mate::Dictionary& options) {
     Show();
 }
 
-gfx::Size NativeWindow::ContentSizeToWindowSize(const gfx::Size& size) {
-  return size;
-}
-
-gfx::Size NativeWindow::WindowSizeToContentSize(const gfx::Size& size) {
-  return size;
-}
-
 void NativeWindow::SetSize(const gfx::Size& size) {
   SetBounds(gfx::Rect(GetPosition(), size));
 }
diff --git a/atom/browser/native_window.h b/atom/browser/native_window.h
index 538783306464..379c23837b19 100644
--- a/atom/browser/native_window.h
+++ b/atom/browser/native_window.h
@@ -242,8 +242,8 @@ class NativeWindow : public base::SupportsUserData,
                const mate::Dictionary& options);
 
   // Converts between content size to window size.
-  virtual gfx::Size ContentSizeToWindowSize(const gfx::Size& size);
-  virtual gfx::Size WindowSizeToContentSize(const gfx::Size& size);
+  virtual gfx::Size ContentSizeToWindowSize(const gfx::Size& size) = 0;
+  virtual gfx::Size WindowSizeToContentSize(const gfx::Size& size) = 0;
 
   // content::WebContentsObserver:
   void RenderViewCreated(content::RenderViewHost* render_view_host) override;