Re: [ServiceWorker] Start with trying to spec foreign fetch. (#749)

@mkruisselbrink thanks for the excellent text. I made minor changes especially in `InstallEvent` and its `event.registerForeignFetchScopes(subScopes)` steps. Note that I changed the parameter type of the method to `sequence<USVString>` in order to maintain consistency with other methods (such as `cache.addAll(sequence<RequestInfo> requests)`) in the spec.

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