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

cc/surfaces/surface_manager.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_MANAGER_H_
 #define CC_SURFACES_SURFACE_MANAGER_H_
 
 #include <stdint.h>
 
 #include <list>
@@ skipping 43 matching lines @@
 
   void RegisterSurfaceIdNamespace(uint32_t id_namespace);
 
   // Invalidate a namespace that might still have associated sequences,
   // possibly because a renderer process has crashed.
   void InvalidateSurfaceIdNamespace(uint32_t id_namespace);
 
  private:
   void GarbageCollectSurfaces();
 
-  using SurfaceMap = std::unordered_map<SurfaceId, Surface*, SurfaceIdHash>;
+  using SurfaceMap = std::unordered_map<SurfaceId, Surface*, SurfaceId::Hasher>;
   SurfaceMap surface_map_;
   base::ObserverList<SurfaceDamageObserver> observer_list_;
   base::ThreadChecker thread_checker_;
 
   // List of surfaces to be destroyed, along with what sequences they're still
   // waiting on.
   using SurfaceDestroyList = std::list<Surface*>;
   SurfaceDestroyList surfaces_to_destroy_;
 
   // Set of SurfaceSequences that have been satisfied by a frame but not yet
   // waited on.
   std::unordered_set<SurfaceSequence, SurfaceSequenceHash> satisfied_sequences_;
 
   // Set of valid surface ID namespaces. When a namespace is removed from
   // this set, any remaining sequences with that namespace are considered
   // satisfied.
   std::unordered_set<uint32_t> valid_surface_id_namespaces_;
 
   DISALLOW_COPY_AND_ASSIGN(SurfaceManager);
 };
 
 }  // namespace cc
 
 #endif  // CC_SURFACES_SURFACE_MANAGER_H_