aura-mus: Implement stacking in DesktopWindowTreeHostMus.

services/ui/public/cpp/tests/test_window_tree.h

Index: services/ui/public/cpp/tests/test_window_tree.h
diff --git a/services/ui/public/cpp/tests/test_window_tree.h b/services/ui/public/cpp/tests/test_window_tree.h
index 594dbd22add6a89c61c63f917eaf580dbd6ec221..1e4c73879881c261f5332ae1fd33819877231d14 100644
--- a/services/ui/public/cpp/tests/test_window_tree.h
+++ b/services/ui/public/cpp/tests/test_window_tree.h
@@ -103,6 +103,7 @@ class TestWindowTree : public mojom::WindowTree {
   void OnWindowInputEventAck(uint32_t event_id,
                              ui::mojom::EventResult result) override;
   void DeactivateWindow(uint32_t window_id) override;
+  void StackAtTop(uint32_t change_id, uint32_t window_id) override;
   void GetWindowManagerClient(
       mojo::AssociatedInterfaceRequest<mojom::WindowManagerClient> internal)
       override;