GL: Don't call seamless cube cap on GLES. 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/546597 Reviewed-by: Yuly Novikov <ynovikov@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>