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This CL connects offline item with the download shelf. Some of
the OfflineItemModel methods are still not implemented.

Bug: 881499
Change-Id: I2f0cd71815a08bcddfca753378543749389d78db
Reviewed-on: https://chromium-review.googlesource.com/1239875
Commit-Queue: Shakti Sahu <shaktisahu@chromium.org>
Reviewed-by: Carlos Knippschild <carlosk@chromium.org>
Reviewed-by: Min Qin <qinmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594471}