ash: Cleanup class/struct forward declarations

ash/common/system/tiles/tiles_default_view.h

 // Copyright 2016 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 ASH_COMMON_SYSTEM_TILES_TILES_DEFAULT_VIEW_H_
 #define ASH_COMMON_SYSTEM_TILES_TILES_DEFAULT_VIEW_H_
 
 #include "ash/ash_export.h"
 #include "ash/common/login_status.h"
 #include "base/macros.h"
 #include "ui/views/controls/button/button.h"
 #include "ui/views/view.h"
 
 namespace views {
 class CustomButton;
 }
 
 namespace ash {
-class SystemMenuButton;
 class SystemTrayItem;
 
 // The container view for the tiles in the bottom row of the system menu
 // (settings, help, lock, and power).
 class ASH_EXPORT TilesDefaultView : public views::View,
                                     public views::ButtonListener {
  public:
   TilesDefaultView(SystemTrayItem* owner, LoginStatus login);
   ~TilesDefaultView() override;
 
@@ skipping 23 matching lines @@
   views::CustomButton* help_button_;
   views::CustomButton* lock_button_;
   views::CustomButton* power_button_;
 
   DISALLOW_COPY_AND_ASSIGN(TilesDefaultView);
 };
 
 }  // namespace ash
 
 #endif  // ASH_COMMON_SYSTEM_TILES_TILES_DEFAULT_VIEW_H_