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This is a reland of b9d68c49
Left the previously disabled test disabled in the reland. Will fix in a
seperate cl.

TBR=oshima@chromium.org

Original change's description:
> splitview: Grab app for splitscreen only happen on long press.
>
> The indicators and the item enlarging only happen after long press, or
> if the item has been dragged.
>
> This is preparation for fling to dismiss. It also removes a flicker of
> the indidcators when you tap to open a item.
>
> Also renables a previous flaky test. Tested locally and it is not flaky
> no more.
>
> Test: ash_unittests SplitViewWindowSelectorTest.*
> Bug: 817863
> Change-Id: I9c20f2fa9a3c0a9218b3072c5b3d25711ad9c655
> Reviewed-on: https://chromium-review.googlesource.com/996528
> Commit-Queue: Sammie Quon <sammiequon@chromium.org>
> Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
> Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#550884}

Bug: 817863
Change-Id: I0d6b255a89e69443af2e6509a0769622c8f64fb3
Reviewed-on: https://chromium-review.googlesource.com/1012735Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551036}