BUG=241
TESTED=J420ToARGB unittest
R=harryjin@google.com

Review URL: https://webrtc-codereview.appspot.com/32929004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@1212 16f28f9a-4ce2-e073-06de-1de4eb20be90