Make HWNDMessageHandler subclass WindowImpl.

ui/views/win/fullscreen_handler.h

Index: ui/views/win/fullscreen_handler.h
===================================================================
--- ui/views/win/fullscreen_handler.h	(revision 153679)
+++ ui/views/win/fullscreen_handler.h	(working copy)
@@ -17,15 +17,13 @@
 
 namespace views {
 
-// TODO(beng): Temporary dependancy until fullscreen moves to
-//             HWNDMessageHandler.
-class Widget;
-
 class FullscreenHandler {
  public:
-  explicit FullscreenHandler(Widget* widget);
+  FullscreenHandler();
   ~FullscreenHandler();
 
+  void set_hwnd(HWND hwnd) { hwnd_ = hwnd; }
+
   void SetFullscreen(bool fullscreen);
   void SetMetroSnap(bool metro_snap);
 
@@ -46,7 +44,7 @@
 
   void SetFullscreenImpl(bool fullscreen, bool for_metro);
 
-  Widget* widget_;
+  HWND hwnd_;
   bool fullscreen_;
   bool metro_snap_;