Bug 214548

Spec: https://wicg.github.io/background-fetch/

Article: https://developers.google.com/web/updates/2018/12/background-fetch

Use case: Fetching (as in down- or uploading) content without the page being open.

Example:

```js
navigator.serviceWorker.ready.then(async (swReg) => {
  const bgFetch = await swReg.backgroundFetch.fetch('my-fetch', ['/ep-5.mp3', 'ep-5-artwork.jpg'], {
    title: 'Episode 5: Interesting things.',
    icons: [{
      sizes: '300x300',
      src: '/ep-5-icon.png',
      type: 'image/png',
    }],
    downloadTotal: 60 * 1024 * 1024,
  });
});
```