aura-mus: Implement stacking in DesktopWindowTreeHostMus.

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 0855369f1ea6fed0d48c4191535b4066150bd6e1..8eeb958b6202ebaf5d4fda2c76ed3b947dc898f1 100644
--- a/ui/aura/mus/window_tree_host_mus.h
+++ b/ui/aura/mus/window_tree_host_mus.h
@@ -71,6 +71,8 @@ class AURA_EXPORT WindowTreeHostMus : public aura::WindowTreeHostPlatform {
   // Requests that the window manager change the activation to the next window.
   void DeactivateWindow();
 
+  void StackAtTop();
+
   // Intended only for WindowTreeClient to call.
   void set_display_id(int64_t id) { display_id_ = id; }
   int64_t display_id() const { return display_id_; }