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The scheduler is now responsible for choosing which
BackgroundFetchJobController runs next. It round-robins through the
list of job controllers launching at most one download from each at a
time. Previously each JobController would get a new request from the
DataManager as soon it completed one, meaning that there would be as
many active downloads as there are JobControllers. The new scheduler
limits downloads globally and so will prevent too many downloads from
happening concurrently.

BackgroundFetchScheduler gets requests from a RequestManager
(implemented by BackgroundFetchDataManager) and delivers them to a
Controller (implemented by BackgroundFetchJobController).

Also eliminates MarkRequestAsStarted and related functions as we don't
do anything with the info.

Bug: 757760
Change-Id: I59f68347e93d278285c83351cbe4e1dd22b91b1b
Reviewed-on: https://chromium-review.googlesource.com/776666
Commit-Queue: Dan Elphick <delphick@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526950}