Add DownloadUIModel

This serves as a base class for us to send UI updates for both download and
offline items

Bug: 881499
Change-Id: I54e50191764f9796250be02c7461cb96b9fa85a6
Reviewed-on: https://chromium-review.googlesource.com/1226456
Commit-Queue: Min Qin <qinmin@chromium.org>
Reviewed-by: Shakti Sahu <shaktisahu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#591505}
6 files changed