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This reverts commit 4d9e568b.

Reason for revert: <INSERT REASONING HERE>

Original change's description:
> shelf: Users cannot hide shelf by swipe down in tablet mode anymore.
> 
> Test: ash_unittests ShelfLayoutManagerTest.SwipingUpOnShelfInTabletModeForFullscreenAppList
> Bug: 801684
> Change-Id: I142180f8204fc413a46381c45267c7be9463c1ef
> Reviewed-on: https://chromium-review.googlesource.com/868453
> Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
> Commit-Queue: Sammie Quon <sammiequon@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#529507}

TBR=xiyuan@chromium.org,sammiequon@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 801684
Change-Id: I5edae712f04a26eea47811e3f2fd69fd0b9c79d9
Reviewed-on: https://chromium-review.googlesource.com/877160Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#530701}