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"
@@ skipping 39 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_ && compositor_->HasAnimationObserver(this)) {

[ajuma, 2015/05/29 21:57:10]

Is HasAnimationObserver necessary here? The call to RemoveAnimationObserver
below winds up in ObserverListBase::RemoveObserver, which doesn't assume the
observe is in the list.

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