aura-mus: Implement stacking in DesktopWindowTreeHostMus.

ui/aura/mus/window_tree_client.h

Index: ui/aura/mus/window_tree_client.h
diff --git a/ui/aura/mus/window_tree_client.h b/ui/aura/mus/window_tree_client.h
index b9e7d9d19613b25fa679a87d8d69451634319919..e057983b548878910fcd577859c7569e9560adb3 100644
--- a/ui/aura/mus/window_tree_client.h
+++ b/ui/aura/mus/window_tree_client.h
@@ -62,6 +62,7 @@ struct WindowPortPropertyData;
 class WindowTreeClientDelegate;
 class WindowTreeClientPrivate;
 class WindowTreeClientObserver;
+class WindowTreeClientTestObserver;
 class WindowTreeHostMus;
 
 namespace client {
@@ -168,6 +169,9 @@ class AURA_EXPORT WindowTreeClient
   void AddObserver(WindowTreeClientObserver* observer);
   void RemoveObserver(WindowTreeClientObserver* observer);
 
+  void AddTestObserver(WindowTreeClientTestObserver* observer);
+  void RemoveTestObserver(WindowTreeClientTestObserver* observer);
+
  private:
   friend class InFlightBoundsChange;
   friend class InFlightFocusChange;
@@ -437,6 +441,7 @@ class AURA_EXPORT WindowTreeClient
       const base::Optional<gfx::Rect>& mask_rect) override;
   void OnWindowTreeHostDeactivateWindow(
       WindowTreeHostMus* window_tree_host) override;
+  void OnWindowTreeHostStackAtTop(WindowTreeHostMus* window_tree_host) override;
   std::unique_ptr<WindowPortMus> CreateWindowPortForTopLevel(
       const std::map<std::string, std::vector<uint8_t>>* properties) override;
   void OnWindowTreeHostCreated(WindowTreeHostMus* window_tree_host) override;
@@ -530,6 +535,8 @@ class AURA_EXPORT WindowTreeClient
 
   std::unique_ptr<DragDropControllerMus> drag_drop_controller_;
 
+  base::ObserverList<WindowTreeClientTestObserver> test_observers_;
+
   std::unique_ptr<ui::Gpu> gpu_;
   std::unique_ptr<MusContextFactory> compositor_context_factory_;
   base::WeakPtrFactory<WindowTreeClient> weak_factory_;