chromeos: Move files in //ash/common to //ash

ash/common/wm_transient_window_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_TRANSIENT_WINDOW_OBSERVER_H_
-#define ASH_COMMON_WM_TRANSIENT_WINDOW_OBSERVER_H_
-
-#include "ash/ash_export.h"
-
-namespace ash {
-
-class WmWindow;
-
-class ASH_EXPORT WmTransientWindowObserver {
- public:
-  virtual void OnTransientChildAdded(WmWindow* window, WmWindow* transient) {}
-  virtual void OnTransientChildRemoved(WmWindow* window, WmWindow* transient) {}
-
- protected:
-  virtual ~WmTransientWindowObserver() {}
-};
-
-}  // namespace ash
-
-#endif  // ASH_COMMON_WM_TRANSIENT_WINDOW_OBSERVER_H_