Recomputed JPeg coefficients normalized to 128.  Apply to ARGBGray function reusing YJ function/coefficients and rounding.
BUG=201
TESTED=Gray unittest improved
Review URL: https://webrtc-codereview.appspot.com/1269006

git-svn-id: http://libyuv.googlecode.com/svn/trunk@629 16f28f9a-4ce2-e073-06de-1de4eb20be90