Move surface reference code to CompositorFrameSinkSupport.

cc/surfaces/referenced_surface_tracker.h

Index: cc/surfaces/referenced_surface_tracker.h
diff --git a/cc/surfaces/referenced_surface_tracker.h b/cc/surfaces/referenced_surface_tracker.h
index 395a853ecbb4343052d24180479247fb211b5e38..bc6e90643bf3c4cfadb457b36320f069606749cb 100644
--- a/cc/surfaces/referenced_surface_tracker.h
+++ b/cc/surfaces/referenced_surface_tracker.h
@@ -26,11 +26,11 @@ class CC_SURFACES_EXPORT ReferencedSurfaceTracker {
 
   const SurfaceId& current_surface_id() const { return current_surface_id_; }
 
-  std::vector<SurfaceReference>& references_to_add() {
+  const std::vector<SurfaceReference>& references_to_add() const {
     return references_to_add_;
   }
 
-  std::vector<SurfaceReference>& references_to_remove() {
+  const std::vector<SurfaceReference>& references_to_remove() const {
     return references_to_remove_;
   }