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

services/ui/ws/window_tree_client_unittest.cc

Index: services/ui/ws/window_tree_client_unittest.cc
diff --git a/services/ui/ws/window_tree_client_unittest.cc b/services/ui/ws/window_tree_client_unittest.cc
index 635891c3904b493017417b429516eff2dab652ae..e06fefb47b684fb081aec31c806294ee7404e31b 100644
--- a/services/ui/ws/window_tree_client_unittest.cc
+++ b/services/ui/ws/window_tree_client_unittest.cc
@@ -476,6 +476,9 @@ class TestWindowTreeClient : public mojom::WindowTreeClient,
   }
   void WmCancelMoveLoop(uint32_t window_id) override { NOTIMPLEMENTED(); }
   void WmDeactivateWindow(uint32_t window_id) override { NOTIMPLEMENTED(); }
+  void WmStackAtTop(uint32_t change_id, uint32_t window_id) override {
+    NOTIMPLEMENTED();
+  }
   void OnAccelerator(uint32_t ack_id,
                      uint32_t accelerator_id,
                      std::unique_ptr<ui::Event> event) override {