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

cc/resources/layer_painter.h

 // Copyright 2011 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_RESOURCES_LAYER_PAINTER_H_
 #define CC_RESOURCES_LAYER_PAINTER_H_
 
 #include "cc/base/cc_export.h"
 
 class SkCanvas;
 
 namespace gfx {
 class Rect;
 class RectF;
 }
 
 namespace cc {
 
 class CC_EXPORT LayerPainter {
  public:
   virtual ~LayerPainter() {}
   virtual void Paint(SkCanvas* canvas,
-                     gfx::Rect content_rect,
+                     const gfx::Rect& content_rect,
                      gfx::RectF* opaque) = 0;
 };
 
 }  // namespace cc
 
 #endif  // CC_RESOURCES_LAYER_PAINTER_H_