Reusing base::IdType<...> to implement SurfaceId.

cc/surfaces/surface_factory.h

 // Copyright 2014 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_FACTORY_H_
 #define CC_SURFACES_SURFACE_FACTORY_H_
 
 #include <set>
 #include <unordered_map>
 
@@ skipping 68 matching lines @@
   void set_needs_sync_points(bool needs) { needs_sync_points_ = needs; }
 
  private:
   SurfaceManager* manager_;
   SurfaceFactoryClient* client_;
   SurfaceResourceHolder holder_;
 
   bool needs_sync_points_;
 
   using OwningSurfaceMap =
-      std::unordered_map<SurfaceId, scoped_ptr<Surface>, SurfaceIdHash>;
+      std::unordered_map<SurfaceId, scoped_ptr<Surface>, SurfaceId::Hasher>;
   OwningSurfaceMap surface_map_;
 
   DISALLOW_COPY_AND_ASSIGN(SurfaceFactory);
 };
 
 }  // namespace cc
 
 #endif  // CC_SURFACES_SURFACE_FACTORY_H_