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

cc/test/layer_tree_test.h

Index: cc/test/layer_tree_test.h
diff --git a/cc/test/layer_tree_test.h b/cc/test/layer_tree_test.h
index a3eaec416f8e71c70b2555c33da77ca5f968d0a9..0a047fa7b49408b3b1687fe8e247d0a1f4bb54a3 100644
--- a/cc/test/layer_tree_test.h
+++ b/cc/test/layer_tree_test.h
@@ -60,7 +60,8 @@ class TestHooks : public AnimationDelegate {
                                     bool has_unfinished_animation) {}
   virtual void WillAnimateLayers(LayerTreeHostImpl* host_impl,
                                  base::TimeTicks monotonic_time) {}
-  virtual void ApplyScrollAndScale(gfx::Vector2d scroll_delta, float scale) {}
+  virtual void ApplyScrollAndScale(const gfx::Vector2d& scroll_delta,
+                                   float scale) {}
   virtual void Animate(base::TimeTicks monotonic_time) {}
   virtual void WillBeginMainFrame() {}
   virtual void DidBeginMainFrame() {}