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Add AshWindowTreeHostMusUnified and AshWindowTreeHostMusMirroringUnified.
(near copies of AshWindowTreeHostUnified and AshWindowTreeHostMirroringUnified)
Make ShellPortMus::CreateAshWindowTreeHost construct these like AshWindowTreeHost::Create.

Loosen DisplayManager::SetDisplayConfiguration restrictions for unified (w/o viz).
Do not show the PlatformDisplay's window for the virtual/offscreen unified display.

TODO: Fix targeting of events on the second display during capture.
TODO: Fix a crash on shutdown when unified mode is still active.

Bug: 764472,770243
TBR: dpranke@chromium.org
Test: Unified mode works reasonably in Mus (needs event handling work)
Change-Id: I25a4f2f917275b4fd9f73cb62a5f22aaf01806ca
Reviewed-on: https://chromium-review.googlesource.com/817761
Commit-Queue: Michael Wasserman <msw@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524186}