Expose the DownloadManager hash to the download service

Pipe the hex encoded hash from the DownloadManager to DownloadService
clients.  The hash may or may not be there due to a variety of reasons
(parallel downloads, reporting the completion after restart, etc.).
However it should be there in the common case.

A follow up patch will allow clients to disable parallel downloads which
will increase the frequency with which the hash is present.

BUG=928816

Change-Id: I01a62575da8eba522b847ee1a204a14926b758d7
Reviewed-on: https://chromium-review.googlesource.com/c/1457673
Commit-Queue: David Trainor <dtrainor@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Xing Liu <xingliu@chromium.org>
Reviewed-by: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632911}
14 files changed