This should improve validation speed slightly because it allows us to
move the ES3-only check into the ES3-only code, and make it a bit
simpler.

BUG=angleproject:747
BUG=angleproject:1390

Change-Id: I41f9ffef1c6a552fde924e62e481831f07b1503a
Reviewed-on: https://chromium-review.googlesource.com/636061Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>