Removes some unnecessary includes, and moves some validatoinfunctions
to the ES3-only file.

BUG=angleproject:1309

Change-Id: I3b274014c48f6f39b5e67223987c91fbc5b4d390
Reviewed-on: https://chromium-review.googlesource.com/636519Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>