Requires a couple tweaks to the back-end to be compatible with the
null driver.

Bug: angleproject:2966
Change-Id: Ia83651aabb0dd14b7d6f64152c276463a8fb28fc
Reviewed-on: https://chromium-review.googlesource.com/c/1392392Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>