Refactor validation to be used in both the OES and ES3 entry points.

BUG=angleproject:600
BUG=angleproject:1101

Change-Id: I2008c4ea04ce07910f03ae0b997f8a77b66203d8
Reviewed-on: https://chromium-review.googlesource.com/316620Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>