[#5] Pass gfx structs by const ref (gfx::Vector2dF)

cc/trees/layer_sorter.cc

 // 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.
 
 #include "cc/trees/layer_sorter.h"
 
 #include <algorithm>
 #include <deque>
 #include <limits>
 #include <vector>
 
 #include "base/logging.h"
 #include "cc/base/math_util.h"
 #include "cc/layers/render_surface_impl.h"
 #include "ui/gfx/transform.h"
 
 namespace cc {
 
 // This epsilon is used to determine if two layers are too close to each other
 // to be able to tell which is in front of the other.  It's a relative epsilon
 // so it is robust to changes in scene scale.  This value was chosen by picking
 // a value near machine epsilon and then increasing it until the flickering on
 // the test scene went away.
 const float k_layer_epsilon = 1e-4f;
 
-inline static float PerpProduct(gfx::Vector2dF u, gfx::Vector2dF v) {
+inline static float PerpProduct(const gfx::Vector2dF& u,
+                                const gfx::Vector2dF& v) {
   return u.x() * v.y() - u.y() * v.x();
 }
 
 // Tests if two edges defined by their endpoints (a,b) and (c,d) intersect.
 // Returns true and the point of intersection if they do and false otherwise.
 static bool EdgeEdgeTest(const gfx::PointF& a,
                          const gfx::PointF& b,
                          const gfx::PointF& c,
                          const gfx::PointF& d,
                          gfx::PointF* r) {
@@ skipping 423 matching lines @@
     *it = sorted_list[count++]->layer;
 
   DVLOG(2) << "Sorting end ----";
 
   nodes_.clear();
   edges_.clear();
   active_edges_.clear();
 }
 
 }  // namespace cc