[ServiceWorker] Is a new property to get the navigation's target client needed? (#817)

As discussed in https://github.com/slightlyoff/ServiceWorker/issues/808, `FetchEvent.clientId` returns null for a navigation request. But as Mozilla suggested, we may need a new property (reservedClientId or potentialClientId?) to get the target client's id if it has use cases.

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Reply to this email directly or view it on GitHub:
https://github.com/slightlyoff/ServiceWorker/issues/817