Re: [ServiceWorker] Promise on navigator.serviceWorker that resolves when page is controlled (#799)

@jeffposnick 

> there could be a new promise, perhaps named .controlled

This could be implemented as:

```js
const p = new Promise(r => {
  if (navigator.serviceWorker.controller) return r();
  navigator.serviceWorker.addEventListener('controllerchange', e => r());
});
```

I think we need more evidence that this is a common enough pattern to add to the platform.

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