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- If there are external downloads active, pause the DownloadService
downloads to not disrupt typical user usage.
- This also means updating DownloadDriver to expose all active downloads
so that on startup the DownloadService can properly set it's state.

Bug: 
Change-Id: Icf3204a1cedafeb9565fc577dc2c4cda3ad283b4
Reviewed-on: https://chromium-review.googlesource.com/537966
Commit-Queue: David Trainor <dtrainor@chromium.org>
Reviewed-by: Xing Liu <xingliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#480290}