Add a new invariant declaration operator. BUG=angle:711 Change-Id: I54a48b636a68c317b8d44ee2d578847b80095289 Reviewed-on: https://chromium-review.googlesource.com/213500 Reviewed-by: Zhenyao Mo <zmo@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>