aura-mus: Implement stacking in DesktopWindowTreeHostMus.

ui/aura/test/mus/test_window_tree.cc

Index: ui/aura/test/mus/test_window_tree.cc
diff --git a/ui/aura/test/mus/test_window_tree.cc b/ui/aura/test/mus/test_window_tree.cc
index aebfee306e74eca029fccf22c9426df7a14995ff..12f94b54e99026e7589f79fcc90d90448bbeba98 100644
--- a/ui/aura/test/mus/test_window_tree.cc
+++ b/ui/aura/test/mus/test_window_tree.cc
@@ -268,6 +268,8 @@ void TestWindowTree::OnWindowInputEventAck(uint32_t event_id,
 
 void TestWindowTree::DeactivateWindow(uint32_t window_id) {}
 
+void TestWindowTree::StackAtTop(uint32_t change_id, uint32_t window_id) {}
+
 void TestWindowTree::GetWindowManagerClient(
     mojo::AssociatedInterfaceRequest<ui::mojom::WindowManagerClient> internal) {
 }