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Use event.location() + ScreenPosition client in FindTargetInRootWindow:
 - location() is in |root_window| coordinates for the ws and clients
 - root_location() is *not* in |root_window| coordinates for clients:
   - |root_window| is the client-root (the top-level client app window)
   - root_location() is Ash's display-root coords (full display bounds)

Add a WindowTreeTest for event locations and root locations.
Add a WindowTargeterTest for FindTargetInRootWindow and regressions.

Bug: 873763, 793916
Test: KSV (Ctrl-Alt-/), Arc, Mash, 2nd display, etc. touch input works.
Change-Id: I7ae541203181f9fc5c65449a10ca927f90d29d96
Reviewed-on: https://chromium-review.googlesource.com/1176462
Commit-Queue: Michael Wasserman <msw@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583488}