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

ash/common/wallpaper/wallpaper_controller_observer.h

Index: ash/common/wallpaper/wallpaper_controller_observer.h
diff --git a/ash/common/wallpaper/wallpaper_controller_observer.h b/ash/common/wallpaper/wallpaper_controller_observer.h
index 2421adfd65a7478c99961bb182353fc57e629103..775c802fd950d1c7d8ac19502f4295e97dcbcf57 100644
--- a/ash/common/wallpaper/wallpaper_controller_observer.h
+++ b/ash/common/wallpaper/wallpaper_controller_observer.h
@@ -14,6 +14,11 @@ class ASH_EXPORT WallpaperControllerObserver {
   // 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() {}
 };