BUG=367
TESTED=unittest on I422ToI420
R=harryjin@google.com

Review URL: https://webrtc-codereview.appspot.com/28639004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@1107 16f28f9a-4ce2-e073-06de-1de4eb20be90