Use a struct for cc::Surface ids for more type safety

cc/surfaces/surface.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_H_
 #define CC_SURFACES_SURFACE_H_
 
 #include "base/macros.h"
 #include "base/memory/scoped_ptr.h"
+#include "cc/surfaces/surface_id.h"
 #include "cc/surfaces/surfaces_export.h"
 #include "ui/gfx/size.h"
 
 namespace cc {
 class CompositorFrame;
 class SurfaceManager;
 class SurfaceClient;
 
 class CC_SURFACES_EXPORT Surface {
  public:
   Surface(SurfaceManager* manager,
           SurfaceClient* client,
           const gfx::Size& size);
   ~Surface();
 
   const gfx::Size& size() const { return size_; }
-  int surface_id() const { return surface_id_; }
+  SurfaceId surface_id() const { return surface_id_; }
 
   void QueueFrame(scoped_ptr<CompositorFrame> frame);
   // Returns the most recent frame that is eligible to be rendered.
   CompositorFrame* GetEligibleFrame();
 
  private:
   SurfaceManager* manager_;
   SurfaceClient* client_;
   gfx::Size size_;
-  int surface_id_;
+  SurfaceId surface_id_;
   // TODO(jamesr): Support multiple frames in flight.
   scoped_ptr<CompositorFrame> current_frame_;
 
   DISALLOW_COPY_AND_ASSIGN(Surface);
 };
 
 }  // namespace cc
 
 #endif  // CC_SURFACES_SURFACE_H_