Optimize QuadF's IsCounterClockwise function

ui/gfx/geometry/quad_f.cc

Index: ui/gfx/geometry/quad_f.cc
diff --git a/ui/gfx/geometry/quad_f.cc b/ui/gfx/geometry/quad_f.cc
index dbc50458b3fa1e04aedfdee3c51d5ae00351b46a..a384ea06dcc53e9c76be8e54ebbb7525a00360de 100644
--- a/ui/gfx/geometry/quad_f.cc
+++ b/ui/gfx/geometry/quad_f.cc
@@ -45,17 +45,16 @@ bool QuadF::IsCounterClockwise() const {
   // pointing downards.
   // Reference: http://mathworld.wolfram.com/PolygonArea.html
 
+  float tmp1 = p2_.y() - p4_.y();

[danakj, 2014/08/11 14:18:49]

how about naming this p24 or something more descriptive than "tmp"?

[kui.zheng, 2014/08/12 04:20:55]

Done.

+  float tmp2 = p3_.y() - p1_.y();

[danakj, 2014/08/11 14:18:49]

ditto

[kui.zheng, 2014/08/12 04:20:55]

Done.

+
   // Up-cast to double so this cannot overflow.
-  double determinant1 = static_cast<double>(p1_.x()) * p2_.y()
-      - static_cast<double>(p2_.x()) * p1_.y();
-  double determinant2 = static_cast<double>(p2_.x()) * p3_.y()
-      - static_cast<double>(p3_.x()) * p2_.y();
-  double determinant3 = static_cast<double>(p3_.x()) * p4_.y()
-      - static_cast<double>(p4_.x()) * p3_.y();
-  double determinant4 = static_cast<double>(p4_.x()) * p1_.y()
-      - static_cast<double>(p1_.x()) * p4_.y();
-
-  return determinant1 + determinant2 + determinant3 + determinant4 < 0;
+  double determinant1 = static_cast<double>(p1_.x()) * tmp1;

[Ian Vollick, 2014/08/11 14:39:09]

If I'm understanding things, determinant1 here is no longer a determinant, but
rather computes some of the terms involved in the old code's sum. If that's
true, I'd vote for something like what Dana suggested above for tmp1 and tmp2
rather than "determinant" and maybe add a comment explaining that what's here is
equivalent to computing the sum of the determinants (i.e., that if you flip the
sign of determinant3 and determinant4 and add them to determinant1 and
determinant2 here, you get the same value as before).

[kui.zheng, 2014/08/12 04:20:55]

Right, This patch just refactoring the computation of adding determinants so
that reducing the number of operations. So, "determinant" is meaningless here. 

+  double determinant2 = static_cast<double>(p2_.x()) * tmp2;
+  double determinant3 = static_cast<double>(p3_.x()) * tmp1;
+  double determinant4 = static_cast<double>(p4_.x()) * tmp2;
+
+  return determinant1 + determinant2 < determinant3 + determinant4;
 }
 
 static inline bool PointIsInTriangle(const PointF& point,