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

cc/layers/surface_layer.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_LAYERS_SURFACE_LAYER_H_
 #define CC_LAYERS_SURFACE_LAYER_H_
 
 #include "cc/base/cc_export.h"
 #include "cc/layers/layer.h"
+#include "cc/surfaces/surface_id.h"
 
 namespace cc {
 
 // A layer that renders a surface referencing the output of another compositor
 // instance or client.
 class CC_EXPORT SurfaceLayer : public Layer {
  public:
   static scoped_refptr<SurfaceLayer> Create();
 
-  void SetSurfaceId(int surface_id);
+  void SetSurfaceId(SurfaceId surface_id);
 
   // Layer overrides.
   virtual scoped_ptr<LayerImpl> CreateLayerImpl(LayerTreeImpl* tree_impl)
       OVERRIDE;
   virtual bool DrawsContent() const OVERRIDE;
   virtual void PushPropertiesTo(LayerImpl* layer) OVERRIDE;
 
  protected:
   SurfaceLayer();
 
  private:
   virtual ~SurfaceLayer();
 
-  int surface_id_;
+  SurfaceId surface_id_;
 
   DISALLOW_COPY_AND_ASSIGN(SurfaceLayer);
 };
 
 }  // namespace cc
 
 #endif  // CC_LAYERS_SURFACE_LAYER_H_