[#3] Pass gfx structs by const ref (gfx::SizeF)

cc/input/layer_scroll_offset_delegate.h

 // Copyright 2013 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_INPUT_LAYER_SCROLL_OFFSET_DELEGATE_H_
 #define CC_INPUT_LAYER_SCROLL_OFFSET_DELEGATE_H_
 
 #include "base/basictypes.h"
 #include "ui/gfx/size_f.h"
 #include "ui/gfx/vector2d_f.h"
@@ skipping 26 matching lines @@
   // This is called by the compositor to check whether a delegate-managed fling
   // is active or not.
   virtual bool IsExternalFlingActive() const = 0;
 
   // This is called by the compositor to notify the delegate what is the current
   // page scale factor is.
   virtual void SetTotalPageScaleFactor(float page_scale_factor) = 0;
 
   // This is called by the compositor to notify the delegate what is the layer's
   // scrollable size is.
-  virtual void SetScrollableSize(gfx::SizeF scrollable_size) = 0;
+  virtual void SetScrollableSize(const gfx::SizeF& scrollable_size) = 0;
 
  protected:
   LayerScrollOffsetDelegate() {}
   virtual ~LayerScrollOffsetDelegate() {}
 
  private:
   DISALLOW_COPY_AND_ASSIGN(LayerScrollOffsetDelegate);
 };
 
 }  // namespace cc
 
 #endif  // CC_INPUT_LAYER_SCROLL_OFFSET_DELEGATE_H_