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This is a reland of c40ba7f2. Added
a missing ASH_EXPORT which was causing ash_unittests to fail.

TBR=xiyuan@chromium.org

Original change's description:
> applist: Introduce HomeLauncherGestureHandler.
>
> Introduce swipe down gesture to homecher. This cl implements the design
> at: https://docs.google.com/document/d/1Bt5956kVDcgUz-_BTJc6M1wm5d03onBgJWE3vHQQGl0/edit#
>
> The design doc states to use a new ScopedClass but after a bit more
> thought I think that would be more difficult especially once animations
> are added.
>
> This should be behind a enabled by default flag, but I do not want to
> merge too much during code review, so I will add the flag shortly
> after this lands.
>
> Test: added tests
> Bug: 872319
> Change-Id: Ibc2dd7344fb3cdb81967b0eefb41d58d377688a3
> Reviewed-on: https://chromium-review.googlesource.com/1184326
> Commit-Queue: Sammie Quon <sammiequon@chromium.org>
> Reviewed-by: Weidong Guo <weidongg@chromium.org>
> Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#585524}

Bug: 872319
Change-Id: Ic3b2320e96cb1fd770ce60a5d48818e7ee78d21c
Reviewed-on: https://chromium-review.googlesource.com/1187042Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585728}