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This is a reland of b513507c

Original change's description:
> tablet: Remove minimum time between taps for quick switch.
> 
> Previously relied on using tap_count in GestureEvent which has a minimum
> time between taps of 40ms for consecutive taps to have tap_count of 2.
> Remove that and track double taps in overview button tray.cc. Allows
> double tap from mice now.
> 
> Test: ash_unittests OverviewButtonTrayTest.*
> Bug: 817883
> Change-Id: Icce2e668223f938a7f8194c5f0a03279abff9907
> Reviewed-on: https://chromium-review.googlesource.com/956769
> Commit-Queue: Sammie Quon <sammiequon@chromium.org>
> Reviewed-by: James Cook <jamescook@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#542315}

Bug: 817883
Change-Id: Ifd6ecabf3f1702d8aeffa29a48a6475f0e6b5422
Reviewed-on: https://chromium-review.googlesource.com/958014Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542553}