Add SurfaceWillDraw notification

cc/surfaces/surface_observer.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_SURFACES_SURFACE_OBSERVER_H_
 #define CC_SURFACES_SURFACE_OBSERVER_H_
 
 namespace cc {
 
 struct BeginFrameAck;
@@ skipping 19 matching lines @@
   virtual bool OnSurfaceDamaged(const SurfaceId& surface_id,
                                 const BeginFrameAck& ack) = 0;
 
   // Called when a surface is garbage-collected.
   virtual void OnSurfaceDiscarded(const SurfaceId& surface_id) = 0;
 
   // Runs when a Surface's CompositorFrame producer has received a BeginFrame
   // 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
 
 #endif  // CC_SURFACES_SURFACE_OBSERVER_H_