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This refines my earlier work done in: http://crrev.com/c/842467

Move event target nulling and root location use to WindowTreeClient, to ensure events reach MusUnifiedEventTargeter.
Otherwise, WindowEventDispatcher::GetInitialEventTarget will use the touch gesture recognizer's target (returned by GetPriorityTargetInRootWindow) before performing the mirror display transformation on the event's location.

Inline DispatchEventToTarget for additional clarity.

Bug: 805714
Test: Mus touch events work in Chrome OS's unified desktop mode.
Change-Id: Ic4844c85cc24a9f9885e7705ff27cb57b305d245
Reviewed-on: https://chromium-review.googlesource.com/887250
Commit-Queue: Michael Wasserman <msw@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532030}