Added LayerAnimationObserver::OnLayerAnimationStarted() notification.

ui/compositor/layer_animation_observer.h

Index: ui/compositor/layer_animation_observer.h
diff --git a/ui/compositor/layer_animation_observer.h b/ui/compositor/layer_animation_observer.h
index 35253d3e3ea335bb0a70f6af0a71e68ef4bd3b73..26bfdfaf0affe45c1578e535741cafe3e9f945e2 100644
--- a/ui/compositor/layer_animation_observer.h
+++ b/ui/compositor/layer_animation_observer.h
@@ -22,6 +22,9 @@ class ImplicitAnimationObserver;
 // LayerAnimationObservers are notified when animations complete.
 class COMPOSITOR_EXPORT LayerAnimationObserver  {
  public:
+  // Called when the |sequence| starts.
+  virtual void OnLayerAnimationStarted(LayerAnimationSequence* sequence);
+
   // Called when the |sequence| ends. Not called if |sequence| is aborted.
   virtual void OnLayerAnimationEnded(
       LayerAnimationSequence* sequence) = 0;