[ash-md] Wired in the Shelf color to be derived from the Wallpaper.

ash/common/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_COMMON_WALLPAPER_WALLPAPER_CONTROLLER_OBSERVER_H_
 #define ASH_COMMON_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;
 
+  // Invoked when the colors extracted from the current wallpaper change. May
+  // be called as a side effect of changing the wallpaper on the
+  // WallpaperController, e.g. WallpaperController::SetWallpaperImage().
+  virtual void OnWallpaperColorsChanged() {}
+
  protected:
   virtual ~WallpaperControllerObserver() {}
 };
 
 }  // namespace ash
 
 #endif  // ASH_COMMON_WALLPAPER_WALLPAPER_CONTROLLER_OBSERVER_H_