[slightlyoff/ServiceWorker] Foreign fetch and opaque origins (#900)

Currently a foreign fetch handler can reply with a non-opaque response by including the origin of the request in the response. If a request was made from an opaque orgin this will never work though, as the origin serialized as string will always be "null", which won't actually be equal to the requests origin.

With regular CORS, a server can always reply with `Access-Control-Allow-Origin: *` to cover unique origins, but currently we don't allow wildcards in foreign fetch handlers. Should wildcards be supported? Or should opaque origins somehow be supported? Or should requests from opaque origins not hit foreign fetch workers at all, since they can't reply to them meaningfully (except if the request was `no-cors`).

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