This is necessary to initialize vertex arrays.

BUG=angleproject:1579

Change-Id: Ic5a232d5cdfaa75b41241901de842e62ff3b173f
Reviewed-on: https://chromium-review.googlesource.com/406645
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>