ash: Cleanup class/struct forward declarations

ash/common/wm_display_observer.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_WM_DISPLAY_OBSERVER_H_
 #define ASH_COMMON_WM_DISPLAY_OBSERVER_H_
 
 #include "ash/ash_export.h"
 
 namespace ash {
 
-class WmWindow;
-
 // Used to track changes in display configuration.
 class ASH_EXPORT WmDisplayObserver {
  public:
   // Called prior to the display configuration changing.
   virtual void OnDisplayConfigurationChanging() {}
 
   // Called after the display configure changed.
   virtual void OnDisplayConfigurationChanged() {}
 
  protected:
   virtual ~WmDisplayObserver() {}
 };
 
 }  // namespace ash
 
 #endif  // ASH_COMMON_WM_DISPLAY_OBSERVER_H_