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This CL introduces the skeleton UI code for OfflineItems.
see https://docs.google.com/document/d/1xDRCoDYnOb9PvwJ8juSmG75jsE-YKzNomCMaoFtQWtQ/edit#

BUG=881499

Change-Id: Ie31f6bcc77c65a4343f2e5a0be54f3b9b5fdd5bc
Reviewed-on: https://chromium-review.googlesource.com/1211445
Commit-Queue: Min Qin <qinmin@chromium.org>
Reviewed-by: Shakti Sahu <shaktisahu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589720}