[ash-md] Remove non-md code from the Shelf background animations.

ash/common/shelf/shelf_background_animator_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_SHELF_SHELF_BACKGROUND_ANIMATOR_OBSERVER_H_
 #define ASH_COMMON_SHELF_SHELF_BACKGROUND_ANIMATOR_OBSERVER_H_
 
 #include "ash/ash_export.h"
 
 namespace ash {
 
 // Observer for the ShelfBackgroundAnimator class.
 class ASH_EXPORT ShelfBackgroundAnimatorObserver {
  public:
   // Called when the Shelf's opaque background should be updated.
   virtual void UpdateShelfOpaqueBackground(int alpha) {}
 
-  // Called when the Shelf's asset based background should be updated.
-  // TODO(bruthig): Remove when non-md is no longer needed (crbug.com/614453).
-  virtual void UpdateShelfAssetBackground(int alpha) {}
-
   // Called when the Shelf item (aka button) backgrounds should be updated.
   virtual void UpdateShelfItemBackground(int alpha) {}
 
  protected:
   virtual ~ShelfBackgroundAnimatorObserver() {}
 };
 
 }  // namespace ash
 
 #endif  // ASH_COMMON_SHELF_SHELF_BACKGROUND_ANIMATOR_OBSERVER_H_