BUG=238
TEST=libyuvTest.ARGBScaleDownBy4_Bilinear
R=dingkai@google.com

Review URL: https://webrtc-codereview.appspot.com/1589004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@711 16f28f9a-4ce2-e073-06de-1de4eb20be90