There was a mistake in one of the texture offset function names being
checked. Fix it and add a test making sure that a similar mistake
cannot slip in again.

BUG=angleproject:2387
TEST=angle_unittests

Change-Id: I2dddfbdc2836f9e7e94ca6573a8f3689eecf7eb2
Reviewed-on: https://chromium-review.googlesource.com/946254Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>