Made the CompositorAnimationObserver class usable

ui/compositor/layer_animator_collection.cc

Index: ui/compositor/layer_animator_collection.cc
diff --git a/ui/compositor/layer_animator_collection.cc b/ui/compositor/layer_animator_collection.cc
index 42dee6c259dd31f0ceb8d03df46bd7ad4daa7de4..6f31e0bb3b0dc8c443948392373ee5871323cfdf 100644
--- a/ui/compositor/layer_animator_collection.cc
+++ b/ui/compositor/layer_animator_collection.cc
@@ -57,4 +57,12 @@ void LayerAnimatorCollection::OnAnimationStep(base::TimeTicks now) {
     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