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Currently the InProgressCache is owned by ChromeDownloadManagerDelegate.
In order for download to work as a service,  the InProgressDownloadManager
should be able to manage InProgressCache. It should read from the cache
and recreate the download before resuming it.
This CL moves the InProgressCache from ChromeDownloadManagerDelegate into
InProgressDownloadmanager. And it also moves StartDownload() method as
this is needed when resuming the download without DownloadManagerImpl.
DownloadManagerImpl now becomes a delegate of InProgressDownloadManager.
It will provide InProgressDownloadManager the information to create a new
download. When resuming a download after launching download service,
DownloadManagerImpl is not needed.

BUG=803135

Change-Id: Ie7a22213e935638dac10dd82d2e079326e8083f2
Reviewed-on: https://chromium-review.googlesource.com/1024723
Commit-Queue: Min Qin <qinmin@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553359}