Add/remove surface references via MojoCompositorFrameSink.

cc/surfaces/surface_reference_manager.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_REFERENCE_MANAGER_H_
 #define CC_SURFACES_SURFACE_REFERENCE_MANAGER_H_
 
 namespace cc {
 
 class SurfaceId;
 
 // Interface to manage surface references.
 class SurfaceReferenceManager {
  public:
   // Returns the top level root SurfaceId. Surfaces that are not reachable
   // from the top level root may be garbage collected. It will not be a valid
   // SurfaceId and will never correspond to a surface.
   virtual const SurfaceId& GetRootSurfaceId() const = 0;
 
   // 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
   // the count is zero nothing is referencing the surface and it may be garbage
   // collected.
   virtual size_t GetSurfaceReferenceCount(
       const SurfaceId& surface_id) const = 0;
 
   // Returns the number of surfaces that |surface_id| has references to.
   virtual size_t GetReferencedSurfaceCount(
       const SurfaceId& surface_id) const = 0;
 
  protected:
   virtual ~SurfaceReferenceManager() {}
 };
 
 }  // namespace cc
 
 #endif  // CC_SURFACES_SURFACE_REFERENCE_MANAGER_H_