[ServiceWorker] Unable to easily distinguish top-level navigations (#673)

In previous iterations of the spec, a property of the `FetchEvent` type allowed the developer to easily understand if they were responding to a potential navigation; e.g. `if(e.isTopLevel) { ...  }`.

This is now gone, nominally replaced by the [`context` attribute of a Request](https://fetch.spec.whatwg.org/#requests).

This, however, doesn't actually provide the same information.

@jakearchibald has made the (cogent) case that a different `Content-Disposition` for a `Response` might not result in a navigation.

I'd like to untangle this and expose what we know about the request. I suggest that [`FetchEvent`](https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#fetch-event-section) be extended with a property that describes if the event corresponds to a potential navigation; straman would be `e.isNavigation`.

*Bonus points*: the `Content-Disposition` handling is magical! It creates the ability for a response to not navigate a frame. This should be an API! In fact, it should be possible to `e.respondWith(/*something*/)` that _explicitly_ has this behavior in order to allow a SW to intercept navigations and drop them on the floor & `postMessage` to the client to effect an in-page transition.

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Reply to this email directly or view it on GitHub:
https://github.com/slightlyoff/ServiceWorker/issues/673