Scale Down by factor tests scale down to specified ratio rather than up.  This ensures the alignment constrains on the destination dont cause a different factor to be used.
BUG=431
TESTED=libyuvTest.ScaleDownBy3_Bilinear
R=harryjin@google.com

Review URL: https://webrtc-codereview.appspot.com/47309004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@1413 16f28f9a-4ce2-e073-06de-1de4eb20be90