[w3c/ServiceWorker] Order of messages to clients in same unit of related browsing contexts unclear (#1355)

The spec doesn't seem to require that messages sent from a service worker to two clients which belong to the same unit of related browsing contexts arrive in the same order they were sent. Without this property it's not possible to observe the state where a client is loading and hasn't called `ServiceWorkerContainer.startMessages()` yet, but has a queued message. This, in turn, makes it impossible to test it with some implementation-specific interface, potentially harming compatibility.

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https://github.com/w3c/ServiceWorker/issues/1355