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This reverts commit 16355cba.

Reason for revert:
This seems to cause an ASAN failure on the CI. The error logs are:
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Linux%20Chromium%20OS%20ASan%20LSan%20Tests%20%281%29/29069
https://logs.chromium.org/logs/chromium/buildbucket/cr-buildbucket.appspot.com/8934902171469785040/+/steps/interactive_ui_tests/0/logs/DownloadNotificationTest.IncognitoDownloadFile/0

Original change's description:
> More OfflineItemModel implementation
> 
> This CL makes OfflineItemModel inherit from DownloadUIModel
> 
> Bug: 881499
> Change-Id: I63270a79917bae87fdb4ae3531f9088f7900509e
> Reviewed-on: https://chromium-review.googlesource.com/1225888
> Commit-Queue: Min Qin <qinmin@chromium.org>
> Reviewed-by: Shakti Sahu <shaktisahu@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#592645}

TBR=qinmin@chromium.org,shaktisahu@chromium.org

Change-Id: Ifdae8934bae3cf137fd2e4729f076f8df577241d
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 881499
Reviewed-on: https://chromium-review.googlesource.com/1234099Reviewed-by: Taiju Tsuiki <tzik@chromium.org>
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592691}