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

ui/compositor/layer_animator.cc

 // Copyright (c) 2012 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.
 
 #include "ui/compositor/layer_animator.h"
 
 #include <stddef.h>
 
 #include <memory>
 
@@ skipping 178 matching lines @@
 
   timeline->DetachPlayer(animation_player_);
 }
 
 void LayerAnimator::AttachLayerToAnimationPlayer(int layer_id) {
   if (!animation_player_->layer_id())
     animation_player_->AttachLayer(layer_id);
   else
     DCHECK_EQ(animation_player_->layer_id(), layer_id);
 
-  if (animation_player_->element_animations()) {
-    animation_player_->element_animations()
-        ->AddEventObserver(this);
-  }
+  if (animation_player_->element_animations())
+    animation_player_->element_animations()->SetEventObserver(this);
 }
 
 void LayerAnimator::DetachLayerFromAnimationPlayer() {
-  if (animation_player_->element_animations()) {
-    animation_player_->element_animations()
-        ->RemoveEventObserver(this);
-  }
+  if (animation_player_->element_animations())
+    animation_player_->element_animations()->SetEventObserver(nullptr);
 
   if (animation_player_->layer_id())
     animation_player_->DetachLayer();
 }
 
 void LayerAnimator::AddThreadedAnimation(
     std::unique_ptr<cc::Animation> animation) {
   animation_player_->AddAnimation(std::move(animation));
 }
 
@@ skipping 744 matching lines @@
     const base::WeakPtr<LayerAnimationSequence>& sequence)
     : sequence_(sequence) {
 }
 
 LayerAnimator::RunningAnimation::RunningAnimation(
     const RunningAnimation& other) = default;
 
 LayerAnimator::RunningAnimation::~RunningAnimation() { }
 
 }  // namespace ui