BUG=197
TESTED=out\release\libyuv_unittest --gtest_filter=*ARGBToI400*
Review URL: https://webrtc-codereview.appspot.com/1176004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@598 16f28f9a-4ce2-e073-06de-1de4eb20be90