BUG=118
TEST=libyuvTest.I420ToI422_OptVsC (247 ms)
Review URL: https://webrtc-codereview.appspot.com/855012

git-svn-id: http://libyuv.googlecode.com/svn/trunk@400 16f28f9a-4ce2-e073-06de-1de4eb20be90