Add two fallbacks for S8_UINT.  Enable tests.

BUG=angleproject:2655

Change-Id: If7df23745a8de8a01d86ab6efa3bca67b5227d76
Reviewed-on: https://chromium-review.googlesource.com/1217282Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>