Made the CompositorAnimationObserver class usable

ui/compositor/compositor_animation_observer.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_COMPOSITOR_ANIMATION_OBSERVER_H_
 #define UI_COMPOSITOR_COMPOSITOR_ANIMATION_OBSERVER_H_
 
 #include "base/time/time.h"
 #include "ui/compositor/compositor_export.h"
 
 namespace ui {
 
+class Compositor;
+
 class COMPOSITOR_EXPORT CompositorAnimationObserver {
  public:
   virtual ~CompositorAnimationObserver() {}
 
   virtual void OnAnimationStep(base::TimeTicks timestamp) = 0;
+  virtual void OnCompositingShuttingDown(Compositor* compositor) = 0;
 };
 
 }  // namespace ui
 
 #endif  // UI_COMPOSITOR_COMPOSITOR_ANIMATION_OBSERVER_H_