Add background fetch to the library

[joeyparrish]

Once we have a PWA (#876), we should look into the use of background sync and/or background fetch to fetch segments for offline storage. This also depends on Fetch support (#829).

With this, one could begin storing content for offline consumption, then close the app and let the segments be fetched and stored in the background, as connectivity is available.

[joeyparrish]

Updated to include both background sync and background fetch as tools we may want to use for this, and closing #881 as a duplicate.

[joeyparrish]

Issues implementing background fetch & sync so far:

• BG fetch is behind a flag in Chrome 61, experimental web platform features must be enabled explicitly. Expected at this stage, but worth noting.
• navigator.serviceWorker provides no consistent indication of service worker existence. Sometimes, controller is null even when a SW is registered. Sometimes, controller becomes non-null after reloading the page. This means that we will need an app-level configuration to instruct us to try BG fetch in the first place, since we can't detect the SW.
• navigator.serviceWorker.ready never resolves on localhost over http. It does on appspot over https. This means we can't reliably grab a reference to the SW registration, even asynchronously. Saving the reference in the app level after initial registration allows me to work around this for development purposes, but this is going to complicate library APIs for BG fetch. If this is consistently the case, we will have to have applications pass in a reference to the SW registration before we can use BG fetch.
• At least on desktop Chrome 61 on Linux, the BG fetch API does not result in a network request. Nothing shows up in the Network debug panel in Chrome.
• At least on desktop Chrome 61 on Linux, the BG fetch API does not provide any way to access the fetch. (May be related to the point immediately above.) The BackgroundFetchRegistration object contains icons, tag, title, and totalDownloadSize. The spec'd activeFetches does not appear. There is no clear way to access the results of the fetch.

Given all of the above, I am moving this issue to the backlog for now.

[joeyparrish]

According to Chrome team, background fetch is not ready for production yet. Leaving this issue on the backlog until that changes.