Add SurfaceWillDraw notification

cc/surfaces/surface_manager.h

Index: cc/surfaces/surface_manager.h
diff --git a/cc/surfaces/surface_manager.h b/cc/surfaces/surface_manager.h
index 01d18b2dc1986ca3c0f22aad6105610916b313a5..6cca04223e82de8af88785e5823fdad5b3afc55f 100644
--- a/cc/surfaces/surface_manager.h
+++ b/cc/surfaces/surface_manager.h
@@ -74,6 +74,10 @@ class CC_SURFACES_EXPORT SurfaceManager {
   // Destroy the Surface once a set of sequence numbers has been satisfied.
   void DestroySurface(std::unique_ptr<Surface> surface);
 
+  // Called when a surface has been added to the aggregated CompositorFrame
+  // and will notify observers with SurfaceObserver::OnSurfaceWillDraw.
+  void SurfaceWillDraw(const SurfaceId& surface_id);
+
   Surface* GetSurfaceForId(const SurfaceId& surface_id);
 
   void AddObserver(SurfaceObserver* obs) { observer_list_.AddObserver(obs); }