compositor: Add rAF-like functionality for the UI.

ui/compositor/layer_animator_collection.cc

Index: ui/compositor/layer_animator_collection.cc
diff --git a/ui/compositor/layer_animator_collection.cc b/ui/compositor/layer_animator_collection.cc
index 12142d88827175352af7545b0924940c5ff6eaa4..0e0fc05b43fb38b8204dc09a4416664ef7233490 100644
--- a/ui/compositor/layer_animator_collection.cc
+++ b/ui/compositor/layer_animator_collection.cc
@@ -13,9 +13,8 @@
 namespace ui {
 
 LayerAnimatorCollection::LayerAnimatorCollection(
-    LayerAnimatorCollectionDelegate* delegate)
-    : delegate_(delegate),
-      last_tick_time_(gfx::FrameTime::Now()) {
+    AnimationFrameTaskProvider* provider)
+    : provider_(provider), last_tick_time_(gfx::FrameTime::Now()) {
 }
 
 LayerAnimatorCollection::~LayerAnimatorCollection() {
@@ -27,21 +26,24 @@ void LayerAnimatorCollection::StartAnimator(
   if (!animators_.size())
     last_tick_time_ = gfx::FrameTime::Now();
   animators_.insert(animator);
-  if (delegate_)
-    delegate_->ScheduleAnimationForLayerCollection();
+  if (!raf_callback_ && provider_)
+    raf_callback_ = provider_->RequestAnimationFrameTask(this);
 }
 
 void LayerAnimatorCollection::StopAnimator(
     scoped_refptr<LayerAnimator> animator) {
   DCHECK_GT(animators_.count(animator), 0U);
   animators_.erase(animator);
+  if (animators_.empty() && raf_callback_)
+    raf_callback_.reset();
 }
 
 bool LayerAnimatorCollection::HasActiveAnimators() const {
   return !animators_.empty();
 }
 
-void LayerAnimatorCollection::Progress(base::TimeTicks now) {
+AnimationFrameTaskStatus LayerAnimatorCollection::Progress(
+    base::TimeTicks now) {
   last_tick_time_ = now;
   std::set<scoped_refptr<LayerAnimator> > list = animators_;
   for (std::set<scoped_refptr<LayerAnimator> >::iterator iter = list.begin();
@@ -51,6 +53,8 @@ void LayerAnimatorCollection::Progress(base::TimeTicks now) {
     if (animators_.count(*iter) > 0)
       (*iter)->Step(now);
   }
+  return HasActiveAnimators() ? ANIMATION_FRAME_TASK_CONTINUE
+                              : ANIMATION_FRAME_TASK_END;
 }
 
 }  // namespace ui