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

cc/output/software_output_device.h

Index: cc/output/software_output_device.h
diff --git a/cc/output/software_output_device.h b/cc/output/software_output_device.h
index 827210e6d0e35019427ddaabf1eb1306abea8d50..f03e96473d10c6d85607d9e1455a10e22565e278 100644
--- a/cc/output/software_output_device.h
+++ b/cc/output/software_output_device.h
@@ -55,8 +55,7 @@ class CC_EXPORT SoftwareOutputDevice {
 
   // Blit the pixel content of the SoftwareOutputDevice by |delta| with the
   // write clipped to |clip_rect|.
-  virtual void Scroll(gfx::Vector2d delta,
-                      const gfx::Rect& clip_rect);
+  virtual void Scroll(const gfx::Vector2d& delta, const gfx::Rect& clip_rect);
 
   // Discard the backing buffer in the surface provided by this instance.
   virtual void DiscardBackbuffer() {}