Added LayerAnimationObserver::OnLayerAnimationStarted() notification.

ui/compositor/layer_animation_sequence.cc

Index: ui/compositor/layer_animation_sequence.cc
diff --git a/ui/compositor/layer_animation_sequence.cc b/ui/compositor/layer_animation_sequence.cc
index 2c01a22d4c04dfb90989dc946486549b7e7f89fb..04a91af2ae3634866bb92c93f5cad70da75754df 100644
--- a/ui/compositor/layer_animation_sequence.cc
+++ b/ui/compositor/layer_animation_sequence.cc
@@ -48,6 +48,8 @@ void LayerAnimationSequence::Start(LayerAnimationDelegate* delegate) {
   if (elements_.empty())
     return;
 
+  NotifyStarted();
+
   elements_[0]->set_requested_start_time(start_time_);
   elements_[0]->Start(delegate, animation_group_id_);
 }
@@ -258,6 +260,11 @@ void LayerAnimationSequence::NotifyScheduled() {
                     OnLayerAnimationScheduled(this));
 }
 
+void LayerAnimationSequence::NotifyStarted() {
+  FOR_EACH_OBSERVER(LayerAnimationObserver, observers_,
+                    OnLayerAnimationStarted(this));
+}
+
 void LayerAnimationSequence::NotifyEnded() {
   FOR_EACH_OBSERVER(LayerAnimationObserver,
                     observers_,