LayerAnimator must be prepared for its owning layer's deletion whenever it notifies observers.

ui/compositor/layer_animation_sequence.h

 // Copyright (c) 2012 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.
 
 #ifndef UI_COMPOSITOR_LAYER_ANIMATION_SEQUENCE_H_
 #define UI_COMPOSITOR_LAYER_ANIMATION_SEQUENCE_H_
 
 #include <vector>
 
 #include "base/gtest_prod_util.h"
@@ skipping 18 matching lines @@
 // sequences.
 class COMPOSITOR_EXPORT LayerAnimationSequence {
  public:
   LayerAnimationSequence();
   // Takes ownership of the given element and adds it to the sequence.
   explicit LayerAnimationSequence(LayerAnimationElement* element);
   virtual ~LayerAnimationSequence();
 
   // Updates the delegate to the appropriate value for |elapsed|, which is in
   // the range [0, Duration()].  If the animation is not aborted, it is
-  // guaranteed that Animate will be called with elapsed = Duration().
-  // Returns true if a redraw is required.
-  bool Progress(base::TimeDelta elapsed, LayerAnimationDelegate* delegate);
+  // guaranteed that Animate will be called with elapsed = Duration(). Requests
+  // a redraw if it is required.
+  void Progress(base::TimeDelta elapsed, LayerAnimationDelegate* delegate);
 
   // Sets the target value to the value that would have been set had
   // the sequence completed. Does nothing if the sequence is cyclic.
   void GetTargetValue(LayerAnimationElement::TargetValue* target) const;
 
   // Aborts the given animation.
   void Abort();
 
   // All properties modified by the sequence.
   const LayerAnimationElement::AnimatableProperties& properties() const {
@@ skipping 62 matching lines @@
 
   // These parties are notified when layer animations end.
   ObserverList<LayerAnimationObserver> observers_;
 
   DISALLOW_COPY_AND_ASSIGN(LayerAnimationSequence);
 };
 
 }  // namespace ui
 
 #endif  // UI_COMPOSITOR_LAYER_ANIMATION_SEQUENCE_H_