[#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 6d282e9a0a0d3deaf233c64b99e323c60cc33054..713699c1302655ccdfac22b27b21f0e427b63824 100644
--- a/cc/test/layer_tree_test.h
+++ b/cc/test/layer_tree_test.h
@@ -59,7 +59,8 @@ 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() {}