ui: Cleanup class/struct forward declarations

ui/views/controls/menu/menu_message_loop.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_VIEWS_CONTROLS_MENU_MENU_MESSAGE_LOOP_H_
 #define UI_VIEWS_CONTROLS_MENU_MENU_MESSAGE_LOOP_H_
 
 #include "ui/gfx/native_widget_types.h"
 
 namespace gfx {
 class Point;
 }
 
 namespace ui {
 class LocatedEvent;
 }
 
 namespace views {
 
 class MenuController;
-class Widget;
 
 // Interface used by MenuController to run a nested message loop while
 // showing a menu, allowing for platform specific implementations.
 class MenuMessageLoop {
  public:
   virtual ~MenuMessageLoop() {}
 
   // Create a platform specific instance.
   static MenuMessageLoop* Create();
 
   // Repost |event| to |window|.
   // |screen_loc| is the event's location in screen coordinates.
   static void RepostEventToWindow(const ui::LocatedEvent* event,
                                   gfx::NativeWindow window,
                                   const gfx::Point& screen_loc);
 
   // Runs the platform specific bits of the message loop.
   virtual void Run() = 0;
 
   // Quit an earlier call to Run().
   virtual void QuitNow() = 0;
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_CONTROLS_MENU_MENU_MESSAGE_LOOP_H_