Remove inaccurate specializations for 1/4 and 3/4, since they round
incorrectly.  Specialize for 100% and 50% are kept due to performance.
Make C and ARM code match SSSE3.
Make unittests expect zero difference.

BUG=libyuv:535
R=harryjin@google.com

Review URL: https://codereview.chromium.org/1533643005 .