Fix Up SingletonHwnd Observer Lifetime Issues

ui/gfx/screen_win.h

Index: ui/gfx/screen_win.h
diff --git a/ui/gfx/screen_win.h b/ui/gfx/screen_win.h
index 60dd39dbdfb98b972581de934e147090113fa2ff..91ccf9de53011fea600edb0782abc1d38a0792ed 100644
--- a/ui/gfx/screen_win.h
+++ b/ui/gfx/screen_win.h
@@ -13,8 +13,7 @@
 
 namespace gfx {
 
-class GFX_EXPORT ScreenWin : public Screen,
-                             public SingletonHwnd::Observer {
+class GFX_EXPORT ScreenWin : public Screen {
  public:
   ScreenWin();
   ~ScreenWin() override;
@@ -33,12 +32,6 @@ class GFX_EXPORT ScreenWin : public Screen,
   void AddObserver(DisplayObserver* observer) override;
   void RemoveObserver(DisplayObserver* observer) override;
 
-  // Overriden from gfx::SingletonHwnd::Observer.
-  void OnWndProc(HWND hwnd,
-                 UINT message,
-                 WPARAM wparam,
-                 LPARAM lparam) override;
-
   // Returns the HWND associated with the NativeView.
   virtual HWND GetHWNDFromNativeView(NativeView window) const;
 
@@ -46,9 +39,13 @@ class GFX_EXPORT ScreenWin : public Screen,
   virtual NativeWindow GetNativeWindowFromHWND(HWND hwnd) const;
 
  private:
+  void OnDisplayChange(HWND hwnd, UINT message, WPARAM wparam, LPARAM lparam);
+
   // Helper implementing the DisplayObserver handling.
   gfx::DisplayChangeNotifier change_notifier_;
 
+  SingletonHwnd::Observer singletonHwndObserver_;
+
   // Current list of displays.
   std::vector<gfx::Display> displays_;