Thread ui opacity animations

ui/compositor/layer_animator.h

Index: ui/compositor/layer_animator.h
diff --git a/ui/compositor/layer_animator.h b/ui/compositor/layer_animator.h
index bf8ee7cfd49bb8207812786d570aff046e3d313b..25cc6781653a2286eaf84f7d05b4a8dbee193218 100644
--- a/ui/compositor/layer_animator.h
+++ b/ui/compositor/layer_animator.h
@@ -113,14 +113,20 @@ class COMPOSITOR_EXPORT LayerAnimator
   // of this animation sequence.
   void ScheduleAnimation(LayerAnimationSequence* animation);
 
-  // Starts the animations to be run together. Obviously will not work if
-  // they animate any common properties. The animator takes ownership of the
+  // Starts the animations to be run together, ensuring that the first elements
+  // in these sequences have the same effective start time even when some of
+  // them start on the compositor thread (but there is no such guarantee for
+  // the effective start time of subsequent elements). Obviously will not work
+  // if they animate any common properties. The animator takes ownership of the
   // animation sequences. Takes PreemptionStrategy into account.
   void StartTogether(const std::vector<LayerAnimationSequence*>& animations);
 
-  // Schedules the animations to be run together. Obviously will not work if
-  // they animate any common properties. The animator takes ownership of the
-  // animation sequences.
+  // Schedules the animations to be run together, ensuring that the first
+  // elements in these sequences have the same effective start time even when
+  // some of them start on the compositor thread (but there is no such guarantee
+  // for the effective start time of subsequent elements). Obviously will not
+  // work if they animate any common properties. The animator takes ownership
+  // of the animation sequences.
   void ScheduleTogether(const std::vector<LayerAnimationSequence*>& animations);
 
   // Schedules a pause for length |duration| of all the specified properties.
@@ -159,6 +165,9 @@ class COMPOSITOR_EXPORT LayerAnimator
   void AddObserver(LayerAnimationObserver* observer);
   void RemoveObserver(LayerAnimationObserver* observer);
 
+  // Called when a threaded animation is actually started.
+  void OnThreadedAnimationStarted(const cc::AnimationEvent& event);
+
   // This determines how implicit animations will be tweened. This has no
   // effect on animations that are explicitly started or scheduled. The default
   // is Tween::LINEAR.