Add new environment settings object field "cross site ancestry" for SameSite cookies work (#1775)

To define how cookies work correctly, particularly around the SameSite attribute, we need to know whether or not an environment settings object has any cross-site ancestors. This is making sure the ESO definition in ServiceWorkers also defines this member algorithm.

See whatwg/html#9000 for even broader context.

Author: bvandersloot-mozilla

docs/index.bs

@@ -3065,6 +3065,8 @@ spec: storage; urlPrefix: https://storage.spec.whatwg.org/
               :: Return |serviceWorker|'s [=service worker/script url=].
               : The [=environment settings object/origin=]
               :: Return its registering [=/service worker client=]'s [=environment settings object/origin=].
+              : The [=environment settings object/has cross-site ancestor=]
+              :: Return its registering [=/service worker client=]'s [=environment settings object/has cross-site ancestor=].
               : The [=environment settings object/policy container=]
               :: Return |workerGlobalScope|'s [=WorkerGlobalScope/policy container=].
               : The [=environment settings object/time origin=]