BUG=279
TEST=ARGBAttenuate_Unaligned
R=nfullagar@google.com, ryanpetrie@google.com

Review URL: https://webrtc-codereview.appspot.com/2709004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@821 16f28f9a-4ce2-e073-06de-1de4eb20be90