The absence of the extension should not prevent depth/stencil textures
in FBOs in ES3, since they are core. In fact it shouldn't be possible
to make depth/stencil attachments at all without support, so the check
shouldn't be necessary.

BUG=angleproject:1828

Change-Id: Ibec1e95c78b79cec9654f06ba11beec447017f1c
Reviewed-on: https://chromium-review.googlesource.com/438604
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>