Tracking bug for the downloads part of the Content Modularization Project.

Downloads is pretty closely tied to networking right now. We should simplify this. Per various discussions, it would be great if:
-downloads is not part of networking, but is instead layered on it
  -network service should only care about fetching requests and giving the response to the caller
-when the browser navigation code notices that the response is a download, it can redirect the data pipe to the download service
-the redesign should be based on mojo for URLLoading ( bug 603396 ), PlzNavigate (bug 689549), and network service (bug 598073)
-download service can be started on its own, with network service, so that downloads can happen without the browser running (bug 695115)