chromeos: Move files in //ash/common to //ash, part 3

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 {
-
-// 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_