cc : Fix occlusion tracking bug for contributing surface

cc/trees/occlusion_tracker.h

Index: cc/trees/occlusion_tracker.h
diff --git a/cc/trees/occlusion_tracker.h b/cc/trees/occlusion_tracker.h
index 9812ff3029a8756ec731030f31e7581ae216e7bd..13ef757917f2758cfd25bc5a4365df5e5dd1cc1f 100644
--- a/cc/trees/occlusion_tracker.h
+++ b/cc/trees/occlusion_tracker.h
@@ -40,6 +40,7 @@ class CC_EXPORT OcclusionTracker {
   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);