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Provided implementation for DownloadOfflineContentProvider::OpenItem. It
will call DownloadItem::OpenDownload and eventually hits
ChromeDownloadManagerDelegate. We will also need to pass DownloadOpenSource
to this interface later for UMA purposes.

Bug: 855727
Change-Id: I86f4f802ab8711ea2162e512e18cbcbf93c1822f
Reviewed-on: https://chromium-review.googlesource.com/1114068
Commit-Queue: Shakti Sahu <shaktisahu@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Cr-Commit-Position: refs/heads/master@{#571580}