Added LayerAnimationObserver::OnLayerAnimationStarted() notification.

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 122 matching lines @@
   friend class LayerAnimatorTestController;
 
   typedef std::vector<linked_ptr<LayerAnimationElement> > Elements;
 
   FRIEND_TEST_ALL_PREFIXES(LayerAnimatorTest,
                            ObserverReleasedBeforeAnimationSequenceEnds);
 
   // Notifies the observers that this sequence has been scheduled.
   void NotifyScheduled();
 
+  // Notifies the observers that this sequence has been started.
+  void NotifyStarted();
+
   // Notifies the observers that this sequence has ended.
   void NotifyEnded();
 
   // Notifies the observers that this sequence has been aborted.
   void NotifyAborted();
 
   // The currently animating element.
   LayerAnimationElement* CurrentElement() const;
 
   // The union of all the properties modified by all elements in the sequence.
@@ skipping 30 matching lines @@
   double last_progressed_fraction_;
 
   base::WeakPtrFactory<LayerAnimationSequence> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(LayerAnimationSequence);
 };
 
 }  // namespace ui
 
 #endif  // UI_COMPOSITOR_LAYER_ANIMATION_SEQUENCE_H_