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..ce4d70874d805d5d8353b029fe00cf589c359e4b 100644
--- a/ui/compositor/layer_animator_collection.cc
+++ b/ui/compositor/layer_animator_collection.cc
@@ -17,7 +17,7 @@ LayerAnimatorCollection::LayerAnimatorCollection(Compositor* compositor)
 }
 
 LayerAnimatorCollection::~LayerAnimatorCollection() {
-  if (compositor_ && compositor_->HasAnimationObserver(this))
+  if (compositor_)
     compositor_->RemoveAnimationObserver(this);
 }
 
@@ -57,4 +57,11 @@ void LayerAnimatorCollection::OnAnimationStep(base::TimeTicks now) {
     compositor_->RemoveAnimationObserver(this);
 }
 
+void LayerAnimatorCollection::OnCompositingShuttingDown(
+    Compositor* compositor) {
+  DCHECK_EQ(compositor_, compositor);
+  compositor_->RemoveAnimationObserver(this);
+  compositor_ = nullptr;
+}
+
 }  // namespace ui