An implementation of BackgroundFetchDelegate has been provided to
content shell which enables Background Fetches, without UI, to finish.
It uses the Download Service through a new dependency. This dependency
will be removed once the Download Service has been servicified, as
//content can depend on it directly from that point forward.

In context of the WPT tests, additional machinery has been added to
enable communication with the Service Worker, required for it to tell
the test about the result of the various Background Fetch events. The
"fetch.https.window.js" file now verifies the behaviour and results
of a single fetched file.

Bug: 864561

Change-Id: If53e6755ae8ea924712f5bdace4df4f658bc53d7
Reviewed-on: https://chromium-review.googlesource.com/1135536
Commit-Queue: Peter Beverloo <[email protected]>
Reviewed-by: Xing Liu <[email protected]>
Reviewed-by: John Abd-El-Malek <[email protected]>
Cr-Commit-Position: refs/heads/master@{#575919}