[WIP] cc: Plumb SurfaceId from clients

ui/compositor/compositor_observer.h

Index: ui/compositor/compositor_observer.h
diff --git a/ui/compositor/compositor_observer.h b/ui/compositor/compositor_observer.h
index b59a01de62a783efd2739a2c682bdf2dc52c98e4..aa2a361003ec5d8d298be83ea3221e1c73ec2e6c 100644
--- a/ui/compositor/compositor_observer.h
+++ b/ui/compositor/compositor_observer.h
@@ -15,6 +15,9 @@ class Compositor;
 // A compositor observer is notified when compositing completes.
 class COMPOSITOR_EXPORT CompositorObserver {
  public:
+  // Called when the accelerated widget is set.
+  virtual void OnCompositingInitialized(Compositor* compositor) = 0;
+
   // A commit proxies information from the main thread to the compositor
   // thread. It typically happens when some state changes that will require a
   // composite. In the multi-threaded case, many commits may happen between