BUG=407
TESTED=ARGBToRGB565Dither unittest
R=harryjin@google.com

Review URL: https://webrtc-codereview.appspot.com/41039004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@1308 16f28f9a-4ce2-e073-06de-1de4eb20be90