Fix depth texture validation in ES2 contexts. 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/186122 Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Shannon Woods <shannonwoods@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>