compositor: Add rAF-like functionality for the UI.

ui/compositor/compositor.cc

Index: ui/compositor/compositor.cc
diff --git a/ui/compositor/compositor.cc b/ui/compositor/compositor.cc
index 603bf487ed24351e78771d66954301522a63d3cb..43a88a6717c3fe1237ef8c6d5d44aa66011c0407 100644
--- a/ui/compositor/compositor.cc
+++ b/ui/compositor/compositor.cc
@@ -289,9 +289,25 @@ bool Compositor::HasObserver(CompositorObserver* observer) {
   return observer_list_.HasObserver(observer);
 }
 
+void Compositor::AddAnimationObserver(CompositorAnimationObserver* observer) {
+  animation_observer_list_.AddObserver(observer);
+  host_->SetNeedsAnimate();
+}
+
+void Compositor::RemoveAnimationObserver(
+    CompositorAnimationObserver* observer) {
+  animation_observer_list_.RemoveObserver(observer);
+}
+
+bool Compositor::HasAnimationObserver(CompositorAnimationObserver* observer) {
+  return animation_observer_list_.HasObserver(observer);
+}
+
 void Compositor::Animate(base::TimeTicks frame_begin_time) {
-  layer_animator_collection_.Progress(frame_begin_time);
-  if (layer_animator_collection_.HasActiveAnimators())
+  FOR_EACH_OBSERVER(CompositorAnimationObserver,
+                    animation_observer_list_,
+                    OnAnimationStep(frame_begin_time));
+  if (animation_observer_list_.might_have_observers())
     host_->SetNeedsAnimate();
 }
 
@@ -360,10 +376,6 @@ void Compositor::DidAbortSwapBuffers() {
                     OnCompositingAborted(this));
 }
 
-void Compositor::ScheduleAnimationForLayerCollection() {
-  host_->SetNeedsAnimate();
-}
-
 const cc::LayerTreeDebugState& Compositor::GetLayerTreeDebugState() const {
   return host_->debug_state();
 }