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In a later CL I'm creating and tearing down the UI widgets for the
touch HUD app, which revealed problems with widget ownership and
child views::View teardown.

* Use a single PointerWatcher across all displays (since PointerWatcher
  tracks events on all displays)
* Fork ash::TouchHudRenderer. The legacy ash one will go away when the
  mojo app becomes the default.
* Make the app own the TouchHudRenderers and TouchHudRenderer own the
  views::Widget.

TODO: Eliminate Launchable interface.

Tbr: sky@chromium.org
Bug: 840380
Test: chrome --show-taps-app paints touch points
Change-Id: Icba099a00f284835099af244db8afd93b9ed7a48
Reviewed-on: https://chromium-review.googlesource.com/1060523Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: kylechar <kylechar@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559304}