[#5] Pass gfx structs by const ref (gfx::Vector2dF)

cc/animation/layer_animation_value_observer.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_ANIMATION_LAYER_ANIMATION_VALUE_OBSERVER_H_
 #define CC_ANIMATION_LAYER_ANIMATION_VALUE_OBSERVER_H_
 
 #include "cc/base/cc_export.h"
 
 namespace cc {
 
 class CC_EXPORT LayerAnimationValueObserver {
  public:
   virtual ~LayerAnimationValueObserver() {}
 
   virtual void OnFilterAnimated(const FilterOperations& filters) = 0;
   virtual void OnOpacityAnimated(float opacity) = 0;
   virtual void OnTransformAnimated(const gfx::Transform& transform) = 0;
-  virtual void OnScrollOffsetAnimated(gfx::Vector2dF scroll_offset) = 0;
+  virtual void OnScrollOffsetAnimated(const gfx::Vector2dF& scroll_offset) = 0;
   virtual void OnAnimationWaitingForDeletion() = 0;
   virtual bool IsActive() const = 0;
 };
 
 }  // namespace cc
 
 #endif  // CC_ANIMATION_LAYER_ANIMATION_VALUE_OBSERVER_H_