Also correct the enum allocation to values that are available to ANGLE.

BUG=angleproject:1635

Change-Id: I443f5654aa6a5049b4a1ae4c253cd6473b4e446e
Reviewed-on: https://chromium-review.googlesource.com/520002
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>