Add/remove surface references via MojoCompositorFrameSink.

cc/surfaces/surface_reference_manager.h

Index: cc/surfaces/surface_reference_manager.h
diff --git a/cc/surfaces/surface_reference_manager.h b/cc/surfaces/surface_reference_manager.h
index 66d55c67a4e96f703ed131bd4341537b90802808..ad85537e67f103da674515825562de0adc68e6b0 100644
--- a/cc/surfaces/surface_reference_manager.h
+++ b/cc/surfaces/surface_reference_manager.h
@@ -19,12 +19,14 @@ class SurfaceReferenceManager {
 
   // Adds a reference from a parent surface to a child surface. Any surface
   // embedding a child surface should have a reference added so that the child
-  // surface is not garbage collected until after the parent surface.
-  virtual void AddSurfaceReference(const SurfaceId& parent_id,
+  // surface is not garbage collected until after the parent surface. Returns
+  // false if the reference was invalid or already exists and wasn't added.
+  virtual bool AddSurfaceReference(const SurfaceId& parent_id,
                                    const SurfaceId& child_id) = 0;
 
-  // Removes a reference from a parent surface to a child surface.
-  virtual void RemoveSurfaceReference(const SurfaceId& parent_id,
+  // Removes a reference from a parent surface to a child surface. Return false
+  // if the reference doesn't exist and wasn't removed.
+  virtual bool RemoveSurfaceReference(const SurfaceId& parent_id,
                                       const SurfaceId& child_id) = 0;
 
   // Returns the number of surfaces that have references to |surface_id|. When