Re: [ServiceWorker] Returning a FetchPromise from fetch() (#625)

I like @tabatkins' ref-count idea. Although I'd only count child abortable promises as refs, `Promise.resolve(fetchp)` would be a simple observer.

So

```
var rootFetchP = fetch(url).then(r => r.json());

var childFetchP1 = rootFetchP.then(data => fetch(data[0]));
var childFetchP2 = rootFetchP.then(data => fetch(data[1]));
var childP = Promise.resolve(rootFetchP).then(r => r.text());

childFetchP1.abort();
// …aborts fetch(data[0]), or waits until it hits that point in the chain, then aborts.
// fetch(url) continues

childFetchP2.abort();
// …aborts fetch(data[1]), or waits until it hits that point in the chain, then aborts.
// fetch(url) aborts also, if not already complete. Out of refs.
// childP rejects (or hangs) as a result
```

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