Add tests for the surfaceless_context extensions BUG=angleproject:1651 Change-Id: Ie486f2f2d0f9adad808265f7f3fbd46069265392 Reviewed-on: https://chromium-review.googlesource.com/434764 Commit-Queue: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>