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This is a reland of c6e9dffe

TBR=oshima@chromium.org

Original change's description:
> overview: Fix how multi touch works for overview drag.
>
> Using two fingers was not restricted which brought a bunch of other bugs.
> Only allow one item to be dragged at a time. Still allows switching of
> fingers midway, so we just check the item which requests drag events is
> the same as the current drag item, instead of checking the gesture
> finger id.
>
> Test: ash_unittests WindowSelectorTest.DraggingWithTwoFingers
> Bug: 827435
> Change-Id: I2c897b1b3c483b5df7a98a658f575a6ebd706acd
> Reviewed-on: https://chromium-review.googlesource.com/988294
> Commit-Queue: Sammie Quon <sammiequon@chromium.org>
> Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#547932}

Bug: 827435
Change-Id: I1cda10e4712f79e458f5b977779809a1ed8255d5
Reviewed-on: https://chromium-review.googlesource.com/996052Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548097}