[#7] Pass gfx structs by const ref (gfx::Size)

cc/layers/heads_up_display_layer.h

 // Copyright 2012 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_HEADS_UP_DISPLAY_LAYER_H_
 #define CC_LAYERS_HEADS_UP_DISPLAY_LAYER_H_
 
 #include <string>
 
 #include "base/memory/scoped_ptr.h"
 #include "cc/base/cc_export.h"
 #include "cc/layers/contents_scaling_layer.h"
 
 namespace cc {
 
 class CC_EXPORT HeadsUpDisplayLayer : public ContentsScalingLayer {
  public:
   static scoped_refptr<HeadsUpDisplayLayer> Create();
 
   void PrepareForCalculateDrawProperties(
-      gfx::Size device_viewport, float device_scale_factor);
+      const gfx::Size& device_viewport, float device_scale_factor);
 
   virtual bool DrawsContent() const OVERRIDE;
 
   virtual scoped_ptr<LayerImpl> CreateLayerImpl(LayerTreeImpl* tree_impl)
       OVERRIDE;
 
  protected:
   HeadsUpDisplayLayer();
 
  private:
   virtual ~HeadsUpDisplayLayer();
 
   DISALLOW_COPY_AND_ASSIGN(HeadsUpDisplayLayer);
 };
 
 }  // namespace cc
 
 #endif  // CC_LAYERS_HEADS_UP_DISPLAY_LAYER_H_