Neon RGBToUV more accurate coefficients and subsample averaging.  Instead of adding 4 pixels and making coefficients 4x smaller, this makes the coefficients 2x small and does a shift, for best accuracy.
BUG=297
TESTED=try bots
R=tpsiaki@google.com, yunqingwang@google.com

Review URL: https://webrtc-codereview.appspot.com/6309004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@920 16f28f9a-4ce2-e073-06de-1de4eb20be90