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

ui/aura/mus/window_tree_client.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_AURA_MUS_WINDOW_TREE_CLIENT_H_
 #define UI_AURA_MUS_WINDOW_TREE_CLIENT_H_
 
 #include <stdint.h>
 
 #include <map>
@@ skipping 391 matching lines @@
       const std::unordered_map<std::string, std::vector<uint8_t>>&
           transport_properties) override;
   void WmClientJankinessChanged(ClientSpecificId client_id,
                                 bool janky) override;
   void WmPerformMoveLoop(uint32_t change_id,
                          Id window_id,
                          ui::mojom::MoveLoopSource source,
                          const gfx::Point& cursor_location) override;
   void WmCancelMoveLoop(uint32_t window_id) override;
   void WmDeactivateWindow(Id window_id) override;
+  void WmStackAtTop(uint32_t change_id, uint32_t window_id) override;
   void OnAccelerator(uint32_t ack_id,
                      uint32_t accelerator_id,
                      std::unique_ptr<ui::Event> event) override;
 
   // Overridden from WindowManagerClient:
   void SetFrameDecorationValues(
       ui::mojom::FrameDecorationValuesPtr values) override;
   void SetNonClientCursor(Window* window, ui::mojom::Cursor cursor_id) override;
   void AddAccelerators(std::vector<ui::mojom::AcceleratorPtr> accelerators,
                        const base::Callback<void(bool)>& callback) override;
@@ skipping 118 matching lines @@
   std::unique_ptr<ui::Gpu> gpu_;
   std::unique_ptr<MusContextFactory> compositor_context_factory_;
   base::WeakPtrFactory<WindowTreeClient> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(WindowTreeClient);
 };
 
 }  // namespace aura
 
 #endif  // UI_AURA_MUS_WINDOW_TREE_CLIENT_H_