Made the CompositorAnimationObserver class usable

ui/compositor/layer_animator_collection.cc

 // 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.
 
 #include "ui/compositor/layer_animator_collection.h"
 
 #include <set>
 
 #include "base/time/time.h"
 #include "ui/compositor/compositor.h"
 #include "ui/compositor/layer_animator.h"
 
 namespace ui {
 
 LayerAnimatorCollection::LayerAnimatorCollection(Compositor* compositor)
     : compositor_(compositor), last_tick_time_(base::TimeTicks::Now()) {
 }
 
 LayerAnimatorCollection::~LayerAnimatorCollection() {
-  if (compositor_ && compositor_->HasAnimationObserver(this))
+  if (compositor_)
     compositor_->RemoveAnimationObserver(this);
 }
 
 void LayerAnimatorCollection::StartAnimator(
     scoped_refptr<LayerAnimator> animator) {
   DCHECK_EQ(0U, animators_.count(animator));
   if (!animators_.size())
     last_tick_time_ = base::TimeTicks::Now();
   animators_.insert(animator);
   if (animators_.size() == 1U && compositor_)
@@ skipping 19 matching lines @@
        iter != list.end();
        ++iter) {
     // Make sure the animator is still valid.
     if (animators_.count(*iter) > 0)
       (*iter)->Step(now);
   }
   if (!HasActiveAnimators() && compositor_)
     compositor_->RemoveAnimationObserver(this);
 }
 
+void LayerAnimatorCollection::OnCompositingShuttingDown(
+    Compositor* compositor) {
+  if (compositor_) {

[danakj, 2015/06/02 18:24:15]

how can compositor_ be null here?

+    compositor_->RemoveAnimationObserver(this);
+  }
+  compositor_ = nullptr;
+}
+
 }  // namespace ui