Add SurfaceWillDraw notification

cc/surfaces/surface_observer.h

Index: cc/surfaces/surface_observer.h
diff --git a/cc/surfaces/surface_observer.h b/cc/surfaces/surface_observer.h
index 5cf1c05225550bc128d13d900a3e38a5a121c45a..a8166a064829dde7152aab233f62e0488704ecde 100644
--- a/cc/surfaces/surface_observer.h
+++ b/cc/surfaces/surface_observer.h
@@ -37,6 +37,9 @@ class SurfaceObserver {
   // and, thus, is expected to produce damage soon.
   virtual void OnSurfaceDamageExpected(const SurfaceId& surface_id,
                                        const BeginFrameArgs& args) = 0;
+
+  // Runs when a surface has been added to the aggregated CompositorFrame.
+  virtual void OnSurfaceWillDraw(const SurfaceId& surface_id) = 0;
 };
 
 }  // namespace cc