BUG=431
TESTED=libyuvTest.ARGBScaleDownBy2_Bilinear

Review URL: https://webrtc-codereview.appspot.com/52569004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@1410 16f28f9a-4ce2-e073-06de-1de4eb20be90