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When mus is not hosting viz, ash is still responsible for managing
the mirroring. So update the MirrorWindowController as appropriate.

Also, do not have mus host viz for ash_unittests --mus.

BUG=786453

Change-Id: I964bb030113f1c0e8a5375056687e2c0ffdba53a
Reviewed-on: https://chromium-review.googlesource.com/808004Reviewed-by: Michael Wasserman <msw@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521760}