mash: Migrate wallpaper controllers to ash/common.

ash/wallpaper/wallpaper_controller_observer.h

-// Copyright (c) 2012 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_WALLPAPER_WALLPAPER_CONTROLLER_OBSERVER_H_
-#define ASH_WALLPAPER_WALLPAPER_CONTROLLER_OBSERVER_H_
-
-#include "ash/ash_export.h"
-
-namespace ash {
-
-class ASH_EXPORT WallpaperControllerObserver {
- public:
-  // Invoked when the wallpaper data is changed.
-  virtual void OnWallpaperDataChanged() = 0;
-
- protected:
-  virtual ~WallpaperControllerObserver() {}
-};
-
-}  // namespace ash
-
-#endif  // ASH_WALLPAPER_WALLPAPER_CONTROLLER_OBSERVER_H_