[w3c/ServiceWorker] should FetchEvent.request.signal reflect abort status of outer request? (#1544)

Consider a service worker script that looks like:

```javascript
  self.addEventListener('fetch', evt => {
    evt.respondWith(fetch(evt.request));
  });
```

And that the controlled page does the following:

```javascript
  const controller = new AbortController();
  fetch(url, { signal: controller.signal });
  if (some_condition) {
    controller.abort();
  }
```

Should the fetch() initiated by the service worker script be aborted in this case?  I think it would be good to do so.

I'm unsure this is what the spec says, though.  It seems that in Handle Fetch we create a `Request` from an inner `request` in step 21.3.2:

https://w3c.github.io/ServiceWorker/#on-fetch-request-algorithm

The fetch spec, however, does not have an abort signal on the inner request:

https://fetch.spec.whatwg.org/#concept-request

Its only present on the exposed `Request` object:

https://fetch.spec.whatwg.org/#request-signal

This implies that Handle Fetch effectively strips the AbortSignal from the request when generating FetchEvent.request.  Is that intentional?

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