This cap is only valid on desktop GL. There might be extensions
where the feature is available as well, so we should follow up
and check for the extensions as well.

This fixes a Debug runtime warning in almost every end2end_test
on GLES.

BUG=angleproject:2085

Change-Id: I5edc1c667b58230df903da82de2a8aceb0369c0c
Reviewed-on: https://chromium-review.googlesource.com/546597Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>