BUG=302
TEST=ARGBScaleDownClipBy2_None
R=tpsiaki@google.com

Review URL: https://webrtc-codereview.appspot.com/6529004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@945 16f28f9a-4ce2-e073-06de-1de4eb20be90