aura-mus: Implement DesktopWindowTreeHostMus::StackAbove().

ui/aura/mus/window_tree_host_mus.h

Index: ui/aura/mus/window_tree_host_mus.h
diff --git a/ui/aura/mus/window_tree_host_mus.h b/ui/aura/mus/window_tree_host_mus.h
index af10e05e45d2b61274a1a9fd1be6dabf073801fa..f028bd286a856e999f3f8d034ed5827575ca3441 100644
--- a/ui/aura/mus/window_tree_host_mus.h
+++ b/ui/aura/mus/window_tree_host_mus.h
@@ -75,6 +75,10 @@ class AURA_EXPORT WindowTreeHostMus : public aura::WindowTreeHostPlatform {
   // Requests that the window manager change the activation to the next window.
   void DeactivateWindow();
 
+  // Requests that our root window be stacked above this other root window
+  // which our connection owns.
+  void StackAbove(Window* window);
+
   // Requests that our root window be stacked above all other parallel root
   // windows which we might not own.
   void StackAtTop();