Added a CallbackLayerAnimationObserver.

ui/compositor/layer_animation_observer.h

Index: ui/compositor/layer_animation_observer.h
diff --git a/ui/compositor/layer_animation_observer.h b/ui/compositor/layer_animation_observer.h
index 26bfdfaf0affe45c1578e535741cafe3e9f945e2..986631672654dd8a83b515ba4a835e9dda6c4cb6 100644
--- a/ui/compositor/layer_animation_observer.h
+++ b/ui/compositor/layer_animation_observer.h
@@ -15,6 +15,10 @@
 
 namespace ui {
 
+namespace test {
+class LayerAnimationObserverTestApi;
+}  // namespace test
+
 class LayerAnimationSequence;
 class ScopedLayerAnimationSettings;
 class ImplicitAnimationObserver;
@@ -46,9 +50,18 @@ class COMPOSITOR_EXPORT LayerAnimationObserver  {
 
   // If the animator is destroyed during an animation, the animations are
   // aborted. The resulting NotifyAborted notifications will NOT be sent to
-  // this observer if this function returns false. NOTE: IF YOU override THIS
-  // FUNCTION TO RETURN TRUE, YOU MUST REMEMBER TO REMOVE YOURSELF AS AN
-  // OBSERVER WHEN YOU ARE DESTROYED.
+  // this observer if this function returns false. An observer who wants to
+  // receive the NotifyAborted notifications during destruction can override
+  // this function to return true.
+  //
+  // *** IMPORTANT ***: If you override this function to return true AND you are
+  // using the pattern described below, then make sure you explicity remove
+  // yourself as an observer before the animator is destroyed!
+  //
+  // This opt-in pattern is used because it is a common pattern for a class to
+  // be the observer of an animation that it itself owns. In such a case the
+  // animator will be destroyed as the owning class is destructed and we don't
+  // want the NotifyAbort being invoked on a partially destroyed observer.
   virtual bool RequiresNotificationWhenAnimatorDestroyed() const;
 
   // Called when |this| is added to |sequence|'s observer list.
@@ -66,6 +79,7 @@ class COMPOSITOR_EXPORT LayerAnimationObserver  {
 
  private:
   friend class LayerAnimationSequence;
+  friend class test::LayerAnimationObserverTestApi;
 
   // Called when |this| is added to |sequence|'s observer list.
   void AttachedToSequence(LayerAnimationSequence* sequence);