BUG=none
TEST=ARGBMultiply ARGBAdd and ARGBSubtract unittests.
Review URL: https://webrtc-codereview.appspot.com/1146006

git-svn-id: http://libyuv.googlecode.com/svn/trunk@588 16f28f9a-4ce2-e073-06de-1de4eb20be90