[#2] Pass gfx structs by const ref (gfx::Rect, gfx::RectF)

cc/layers/content_layer_client.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_CONTENT_LAYER_CLIENT_H_
 #define CC_LAYERS_CONTENT_LAYER_CLIENT_H_
 
 #include "cc/base/cc_export.h"
 
 class SkCanvas;
 
 namespace gfx {
 class Rect;
 class RectF;
 }
 
 namespace cc {
 
 class CC_EXPORT ContentLayerClient {
  public:
   virtual void PaintContents(SkCanvas* canvas,
-                             gfx::Rect clip,
+                             const gfx::Rect& clip,
                              gfx::RectF* opaque) = 0;
 
   // Called by the content layer during the update phase.
   // If the client paints LCD text, it may want to invalidate the layer.
   virtual void DidChangeLayerCanUseLCDText() = 0;
 
  protected:
   virtual ~ContentLayerClient() {}
 };
 
 }  // namespace cc
 
 #endif  // CC_LAYERS_CONTENT_LAYER_CLIENT_H_