Similar to prior fix for GLES2, validation now correctly returns
INVALID_ENUM instead of shorting out.

BUG=angleproject:1168

Change-Id: I8247b0b3520b0f116856049c85606bb44fa24021
Reviewed-on: https://chromium-review.googlesource.com/303472Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tryjob-Request: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Tommy Martino <tmartino@chromium.org>