CC Animation: Make LayerAnimationController to have just one event observer.

cc/animation/element_animations.h

Index: cc/animation/element_animations.h
diff --git a/cc/animation/element_animations.h b/cc/animation/element_animations.h
index 4c69014f40383eb3bce2a76ce1d0d2ccf898dfe9..72051e3e8e7c0642f5c4c33039398c1236e49176 100644
--- a/cc/animation/element_animations.h
+++ b/cc/animation/element_animations.h
@@ -90,8 +90,7 @@ class CC_EXPORT ElementAnimations : public base::RefCounted<ElementAnimations>,
   // Returns the active animation for the given unique animation id.
   Animation* GetAnimationById(int animation_id) const;
 
-  void AddEventObserver(LayerAnimationEventObserver* observer);
-  void RemoveEventObserver(LayerAnimationEventObserver* observer);
+  void SetEventObserver(LayerAnimationEventObserver* observer);
 
  private:
   friend class base::RefCounted<ElementAnimations>;