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

cc/layers/layer.h

Index: cc/layers/layer.h
diff --git a/cc/layers/layer.h b/cc/layers/layer.h
index 5f9caf6eef0d2c236b3cfccbc73aabf8a9b08c08..fe16dc050cf7909e7d1b4eabdb3855e4bd31d51d 100644
--- a/cc/layers/layer.h
+++ b/cc/layers/layer.h
@@ -265,11 +265,11 @@ class CC_EXPORT Layer : public base::RefCounted<Layer>,
     return draw_properties_.num_unclipped_descendants;
   }
 
-  void SetScrollOffset(gfx::Vector2d scroll_offset);
+  void SetScrollOffset(const gfx::Vector2d& scroll_offset);
   gfx::Vector2d scroll_offset() const { return scroll_offset_; }
-  void SetScrollOffsetFromImplSide(gfx::Vector2d scroll_offset);
+  void SetScrollOffsetFromImplSide(const gfx::Vector2d& scroll_offset);
 
-  void SetMaxScrollOffset(gfx::Vector2d max_scroll_offset);
+  void SetMaxScrollOffset(const gfx::Vector2d& max_scroll_offset);
   gfx::Vector2d max_scroll_offset() const { return max_scroll_offset_; }
 
   void SetScrollable(bool scrollable);