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

ui/aura/mus/window_tree_client.h

Index: ui/aura/mus/window_tree_client.h
diff --git a/ui/aura/mus/window_tree_client.h b/ui/aura/mus/window_tree_client.h
index e057983b548878910fcd577859c7569e9560adb3..486a06371be114c4e6f1f285d6a57f5a4b14dadb 100644
--- a/ui/aura/mus/window_tree_client.h
+++ b/ui/aura/mus/window_tree_client.h
@@ -409,6 +409,7 @@ class AURA_EXPORT WindowTreeClient
                          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;
@@ -510,6 +511,9 @@ class AURA_EXPORT WindowTreeClient
 
   bool in_destructor_;
 
+  // Maps from a change id to a window manager change id which caused it.
+  std::map<uint32_t, uint32_t> in_flight_change_map_;
+
   // A mapping to shared memory that is one 32 bit integer long. The window
   // server uses this to let us synchronously read the cursor location.
   mojo::ScopedSharedBufferMapping cursor_location_mapping_;