aura-mus: Implement stacking in DesktopWindowTreeHostMus.

ui/views/mus/mus_client.h

Index: ui/views/mus/mus_client.h
diff --git a/ui/views/mus/mus_client.h b/ui/views/mus/mus_client.h
index 075450b04f050a954fa752118cd1958fdb384f21..59d083144a50810116d0788e72eb132118575ee6 100644
--- a/ui/views/mus/mus_client.h
+++ b/ui/views/mus/mus_client.h
@@ -59,6 +59,7 @@ class NativeWidgetDelegate;
 
 namespace test {
 class MusClientTestApi;
+class MusClientTestObserver;
 }
 
 // MusClient establishes a connection to mus and sets up necessary state so that
@@ -122,6 +123,7 @@ class VIEWS_MUS_EXPORT MusClient
   void OnEmbedRootDestroyed(aura::WindowTreeHostMus* window_tree_host) override;
   void OnPointerEventObserved(const ui::PointerEvent& event,
                               aura::Window* target) override;
+  void OnChangeCompleted(aura::ChangeType type, bool success) override;
   aura::client::CaptureClient* GetCaptureClient() override;
   aura::PropertyConverter* GetPropertyConverter() override;
 
@@ -140,6 +142,9 @@ class VIEWS_MUS_EXPORT MusClient
 
   base::ObserverList<MusClientObserver> observer_list_;
 
+  // An observer that's only set during tests to listen for internal events.
+  test::MusClientTestObserver* test_observer_;
+
   // NOTE: this may be null (creation is based on argument supplied to
   // constructor).
   std::unique_ptr<wm::WMState> wm_state_;