We were checking for extra conditions that apply to ES3 within all
ES2 contexts, causing the webgl depth texture conformance test to
fail.

BUG=angle:550

Change-Id: Iae9a4880b9efc6be8c6857bce27a47d42fecc23a
Reviewed-on: https://chromium-review.googlesource.com/186122Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>