This feature adds an `id` attribute to ServiceWorkerRegistration which acts as a unique key identifying the registration. Previously the registration's scope was used as the unique key. By moving to a separate `id` value the scope can now be changed atomically without unregistering and re-registering the existing registration.
This change adds a `ServiceWorkerRegistration.id` attribute which can be used to uniquely identify registrations. This allows us to make the registration's scope mutable. Previously if you wanted to change the scope you would have to unregister and re-register the service worker which could lose state like push subscriptions, etc.
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