aura-mus: Make StackAtTop() proxy to the window manager.

services/ui/ws/test_utils.cc

Index: services/ui/ws/test_utils.cc
diff --git a/services/ui/ws/test_utils.cc b/services/ui/ws/test_utils.cc
index 38df6fa8d054985a91b2e1c85346371230ae3adb..c89d695dd19395a0ab0cbe3e31171c0fc7f76b96 100644
--- a/services/ui/ws/test_utils.cc
+++ b/services/ui/ws/test_utils.cc
@@ -247,6 +247,8 @@ void TestWindowManager::WmCancelMoveLoop(uint32_t window_id) {}
 
 void TestWindowManager::WmDeactivateWindow(uint32_t window_id) {}
 
+void TestWindowManager::WmStackAtTop(uint32_t change_id, uint32_t window_id) {}
+
 void TestWindowManager::OnAccelerator(uint32_t ack_id,
                                       uint32_t accelerator_id,
                                       std::unique_ptr<ui::Event> event) {