Revert "shelf: Users cannot hide shelf by swipe down in tablet mode anymore."

This reverts commit 4d9e568ba40d64bdab2be7c0604cdebfeb6162c2.

Original commit landed in 65 and was then reverted, but revert missed 65.

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


(cherry picked from commit 313181911aa1d84f51d3e0cbab325426d0b9ab2b)

Bug: 801684
Change-Id: I5edae712f04a26eea47811e3f2fd69fd0b9c79d9
Reviewed-on: https://chromium-review.googlesource.com/877160
Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#530701}
Reviewed-on: https://chromium-review.googlesource.com/941735
Cr-Commit-Position: refs/branch-heads/3325@{#629}
Cr-Branched-From: bc084a8b5afa3744a74927344e304c02ae54189f-refs/heads/master@{#530369}
2 files changed