Made the CompositorAnimationObserver class usable

ui/compositor/layer_animator_collection.h

 // Copyright 2014 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 UI_COMPOSITOR_LAYER_ANIMATOR_COLLECTION_H_
 #define UI_COMPOSITOR_LAYER_ANIMATOR_COLLECTION_H_
 
 #include <set>
 
 #include "base/callback.h"
@@ skipping 17 matching lines @@
 
   void StartAnimator(scoped_refptr<LayerAnimator> animator);
   void StopAnimator(scoped_refptr<LayerAnimator> animator);
 
   bool HasActiveAnimators() const;
 
   base::TimeTicks last_tick_time() const { return last_tick_time_; }
 
   // CompositorAnimationObserver:
   void OnAnimationStep(base::TimeTicks timestamp) override;
+  void OnCompositingShuttingDown(Compositor* compositor) override;
 
  private:
   Compositor* compositor_;
   base::TimeTicks last_tick_time_;
   std::set<scoped_refptr<LayerAnimator> > animators_;
 
   DISALLOW_COPY_AND_ASSIGN(LayerAnimatorCollection);
 };
 
 }  // namespace ui
 
 #endif  // UI_COMPOSITOR_LAYER_ANIMATOR_COLLECTION_H_