BUG=201
TESTED=out\release\libyuv_unittest --gtest_filter=*Sobel*
Review URL: https://webrtc-codereview.appspot.com/1225004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@614 16f28f9a-4ce2-e073-06de-1de4eb20be90