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Create one touch hud views::Widget per display and open/close new
widgets as displays are added and removed.

Bug: 840380
Test: run chrome --show-taps-app and add/remove displays
Change-Id: I21c87b35ec520bd1501f0cbf02c073d030c7ff64
Reviewed-on: https://chromium-review.googlesource.com/1058591
Commit-Queue: James Cook <jamescook@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558731}