cc : Fix occlusion tracking bug for contributing surface

cc/trees/occlusion_tracker.h

 // Copyright 2012 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_TREES_OCCLUSION_TRACKER_H_
 #define CC_TREES_OCCLUSION_TRACKER_H_
 
 #include <vector>
 
 #include "base/macros.h"
@@ skipping 22 matching lines @@
   explicit OcclusionTracker(const gfx::Rect& screen_space_clip_rect);
   ~OcclusionTracker();
 
   // Return an occlusion that retains the current state of the tracker
   // and can be used outside of a layer walk to check occlusion.
   Occlusion GetCurrentOcclusionForLayer(
       const gfx::Transform& draw_transform) const;
   Occlusion GetCurrentOcclusionForContributingSurface(
       const gfx::Transform& draw_transform) const;
 
+  const RenderSurfaceImpl* OcclusionSurfaceForContributingSurface() const;
   // Called at the beginning of each step in the LayerIterator's front-to-back
   // traversal.
   void EnterLayer(const LayerIteratorPosition& layer_iterator);
   // Called at the end of each step in the LayerIterator's front-to-back
   // traversal.
   void LeaveLayer(const LayerIteratorPosition& layer_iterator);
 
   // Gives the region of the screen that is not occluded by something opaque.
   Region ComputeVisibleRegionInScreen(const LayerTreeImpl* layer_tree) const;
 
@@ skipping 48 matching lines @@
 
   gfx::Rect screen_space_clip_rect_;
   gfx::Size minimum_tracking_size_;
 
   DISALLOW_COPY_AND_ASSIGN(OcclusionTracker);
 };
 
 }  // namespace cc
 
 #endif  // CC_TREES_OCCLUSION_TRACKER_H_