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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}