[w3c/ServiceWorker] Job queue items may not have a service worker registration (#1180)

There's a separate [job queue](https://w3c.github.io/ServiceWorker/#dfn-job-queue) for each service worker registration, but during the initial call to `serviceWorker.register()` that registration does not exist, yet a job is queued in [Start Register](https://w3c.github.io/ServiceWorker/#start-register-algorithm).

If there are two initial calls to register for the same scope in different processes, we want to ensure they queue.

@jungkees what do you think? Seems like we can safely remove the reference to the registration in the [job queue](https://w3c.github.io/ServiceWorker/#dfn-job-queue), and use scope url as the key.

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