Only expose the EGL robutness extensions when the native GL context has them. On some drivers, the WGL_ARB_create_context_robustness extension is present but the created context does not expose the extensions. Be more defensive and only expose robustness if we're able to create contexts that have it. BUG=angleproject:1538 Change-Id: I3d02b2071c7b2c576abe66cc283cee467e400fd3 Reviewed-on: https://chromium-review.googlesource.com/391833 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: John Bauman <jbauman@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>