Implement textureGradOffset. BUG=angle:564 Change-Id: Iad0e452f53469208d55755e14a350b5d2ca83e9d Reviewed-on: https://chromium-review.googlesource.com/186987 Tested-by: Nicolas Capens <nicolascapens@chromium.org> Reviewed-by: Shannon Woods <shannonwoods@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>