Use an auxiliary list of animation controllers.

cc/animation_registrar.h

+// Copyright 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 CC_ANIMATION_REGISTRAR_H_
+#define CC_ANIMATION_REGISTRAR_H_
+
+namespace cc {
+
+class LayerAnimationController;
+
+class CC_EXPORT AnimationRegistrar {
+ public:
+  virtual void Register(LayerAnimationController*) = 0;
+  virtual void Unregister(LayerAnimationController*) = 0;
+};
+
+}  // namespace cc
+
+#endif  // CC_ANIMATION_REGISTRAR_H_