Add gl null driver tests to DrawElements. 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/1392392 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>