Prevents format validation errors with WebGL context.

BUG=angleproject:1523

Change-Id: Iddc525eeb467de0139e166dad0893f3bea3ef35f
Reviewed-on: https://chromium-review.googlesource.com/650807
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>