[w3c/ServiceWorker] Formalizing a timeout API (#1292)

It looks like the idea has [already been brought up](https://github.com/w3c/ServiceWorker/issues/980#issuecomment-314487339) and is [already implemented](https://github.com/w3c/ServiceWorker/issues/1182) in some fashion in at least FF and chrome, but it would be nice is there was some spec guarantee on guarding async operations with a timeout, and some way to customize behavior.

In theory, it's not hard to create a service worker than can never be removed, by creating some aysnc operation that can never finish:

```JavaScript
e.waitUntil(new Promise(() => {}));
```
```JavaScript
addEventListener('install', (e) => {
  e.waitUntil(registration.unregister());
});
```

A few ideas for public APIs to prevent these issues and provide fine-grain control:
#### Additional timeout param for `waitUntil`/`respondWith`
```JavaScript
// Abort if promise doesn't resolve within 5 seconds.
e.waitUntil(new Promise(() => {}), { timeout: 5000 });
```

#### API for adding a default timeout for functional events
Not quite sure where this could be done. Perhaps during worker registration? I feel like adding a worker specific option to `addEventListener` would be too intrusive.

#### Allow installing worker to cutoff active worker
Specifically, allow `skipWaiting` to provide an “ultimatum” timeout to the active worker to finish any async operations.

```JavaScript
// Give the active worker 5 seconds to terminate any operations before killing
self.skipWaiting({
  gracePeriod: 5000
});
```


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