Add skeleton for DisplayNSGL and WindowSurfaceNSGL BUG=angleproject:891 Change-Id: Ie79e76c68775e1144ac224a4f1eba85f8a889269 Reviewed-on: https://chromium-review.googlesource.com/286829 Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Kenneth Russell <kbr@chromium.org> Tested-by: Corentin Wallez <cwallez@chromium.org>