ARGBGrayTo takes source and destination pointers so caller does not need to make a copy of the image.
BUG=none
TEST=none
Review URL: https://webrtc-codereview.appspot.com/696004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@308 16f28f9a-4ce2-e073-06de-1de4eb20be90