BUG=260
TEST=ScaleDownByVertical2by3_Bilinear
R=jingning@google.com, thorcarpenter@google.com

Review URL: https://webrtc-codereview.appspot.com/2165004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@779 16f28f9a-4ce2-e073-06de-1de4eb20be90