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..85fdd02c1b4861c809393a09d6e99b2b23376c6b 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 SurfaceAggregator has added the surface to the Display Frame.

[danakj, 2017/06/14 18:53:12]

same here re Display Frame, this is about SurfaceAggregator including the
Surface in what is going to be drawn (which is represented as a CompositorFrame
right now, tho may not always)

[gklassen, 2017/06/14 19:29:11]

Agreed.  Comment updated. 

Done.

+  virtual void OnSurfaceWillDraw(const SurfaceId& surface_id) = 0;
 };
 
 }  // namespace cc