[#6]Pass gfx structs by const ref (gfx::Vector2d)

cc/trees/layer_tree_host.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_TREES_LAYER_TREE_HOST_H_
 #define CC_TREES_LAYER_TREE_HOST_H_
 
 #include <limits>
 #include <list>
 #include <set>
@@ skipping 217 matching lines @@
 
   void SetOverhangBitmap(const SkBitmap& bitmap);
 
   PrioritizedResourceManager* contents_texture_manager() const {
     return contents_texture_manager_.get();
   }
 
   void SetVisible(bool visible);
   bool visible() const { return visible_; }
 
-  void StartPageScaleAnimation(gfx::Vector2d target_offset,
+  void StartPageScaleAnimation(const gfx::Vector2d& target_offset,
                                bool use_anchor,
                                float scale,
                                base::TimeDelta duration);
 
   void ApplyScrollAndScale(const ScrollAndScaleSet& info);
   gfx::Vector2d DistributeScrollOffsetToViewports(const gfx::Vector2d offset,
                                                   Layer* layer);
 
   void SetImplTransform(const gfx::Transform& transform);
 
@@ skipping 211 matching lines @@
 
   ScopedPtrVector<SwapPromise> swap_promise_list_;
   std::set<SwapPromiseMonitor*> swap_promise_monitor_;
 
   DISALLOW_COPY_AND_ASSIGN(LayerTreeHost);
 };
 
 }  // namespace cc
 
 #endif  // CC_TREES_LAYER_TREE_HOST_H_