Implement service-side surface synchronization

cc/surfaces/surface_id_allocator.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_ID_ALLOCATOR_H_
 #define CC_SURFACES_SURFACE_ID_ALLOCATOR_H_
 
 #include <stdint.h>
 
 #include "base/macros.h"
 #include "cc/surfaces/surface_id.h"
 #include "cc/surfaces/surfaces_export.h"
 
 namespace cc {
 
 // This is a helper class for generating surface IDs within a specified
 // namespace.  This is not threadsafe, to use from multiple threads wrap this
 // class in a mutex.
 class CC_SURFACES_EXPORT SurfaceIdAllocator {
  public:
   SurfaceIdAllocator();
   ~SurfaceIdAllocator();
 
-  LocalSurfaceId GenerateId();
+  const LocalSurfaceId& GenerateId();
+
+  const LocalSurfaceId& current_local_surface_id() const {
+    return current_local_surface_id_;
+  }
 
  private:
   uint32_t next_id_;
+  LocalSurfaceId current_local_surface_id_;
 
   DISALLOW_COPY_AND_ASSIGN(SurfaceIdAllocator);
 };
 
 }  // namespace cc
 
 #endif  // CC_SURFACES_SURFACE_ID_ALLOCATOR_H_