Define settings object's cross-origin isolated capability for SW

Related to whatwg/html#5435 and
whatwg/html#5752.

Author: yutakahirano

docs/index.bs

@@ -2894,6 +2894,8 @@ spec: webappsec-referrer-policy; urlPrefix: https://w3c.github.io/webappsec-refe
               :: Return |workerGlobalScope|'s [=WorkerGlobalScope/referrer policy=].
               : The [=environment settings object/embedder policy=]
               :: Return |workerGlobalScope|'s [=WorkerGlobalScope/embedder policy=].
+              : The [=environment settings object/cross-origin isolated capability=]
+              :: Return whether |workerGlobalScope|'s [=WorkerGlobalScope/embedder policy=]'s [=embedder policy/value] is "`[embedder policy value/require-corp]`".
 
           1. Set |settingsObject|'s [=environment/id=] to a new unique opaque string, [=creation URL=] to |serviceWorker|'s [=service worker/script url=], [=environment/top-level creation URL=] to null, [=environment/top-level origin=] to an [=implementation-defined=] value, [=environment/target browsing context=] to null, and [=active service worker=] to null.
           1. Set |workerGlobalScope|'s [=WorkerGlobalScope/url=] to |serviceWorker|'s [=service worker/script url=].