This stops Mesa from complaining when using GLES in the back end.

BUG=angleproject:1301

Change-Id: I0b8cb78efea97c1ea0d21cd6b9f656e53b9b6cc2
Reviewed-on: https://chromium-review.googlesource.com/331395Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>