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

cc/animation/element_animations.h

 // Copyright 2015 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 CC_ANIMATION_ELEMENT_ANIMATIONS_H_
 #define CC_ANIMATION_ELEMENT_ANIMATIONS_H_
 
 #include <memory>
 
 #include "base/containers/linked_list.h"
@@ skipping 72 matching lines @@
   void AbortAnimations(TargetProperty::Type target_property,
                        bool needs_completion = false);
 
   // Returns the active animation animating the given property that is either
   // running, or is next to run, if such an animation exists.
   Animation* GetAnimation(TargetProperty::Type target_property) const;
 
   // 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>;
 
   // TODO(loyso): Erase this when LAC merged into ElementAnimations.
   friend class AnimationHost;
 
   explicit ElementAnimations(AnimationHost* host);
   ~ElementAnimations() override;
 
@@ skipping 38 matching lines @@
   // LAC is owned by ElementAnimations (1:1 relationship).
   scoped_refptr<LayerAnimationController> layer_animation_controller_;
   AnimationHost* animation_host_;
 
   DISALLOW_COPY_AND_ASSIGN(ElementAnimations);
 };
 
 }  // namespace cc
 
 #endif  // CC_ANIMATION_ELEMENT_ANIMATIONS_H_