Allow parallel creation of windows

services/ui/demo/mus_demo_external.h

Index: services/ui/demo/mus_demo_external.h
diff --git a/services/ui/demo/mus_demo_external.h b/services/ui/demo/mus_demo_external.h
index 844e04186fd225084c1cbcd2cf7c175ce6de503c..5e662f4f0d6a12e96a39e9608bb95f8f6fe1259f 100644
--- a/services/ui/demo/mus_demo_external.h
+++ b/services/ui/demo/mus_demo_external.h
@@ -22,18 +22,16 @@ class MusDemoExternal : public MusDemo {
   ~MusDemoExternal() final;
 
  private:
-  void OpenNewWindow();
+  void OpenNewWindow(size_t window_index);
 
   // ui::demo::MusDemo:
   void OnStartImpl() final;
   std::unique_ptr<aura::WindowTreeClient> CreateWindowTreeClient() final;
 
   // aura::WindowTreeClientDelegate:
-  void OnEmbed(std::unique_ptr<aura::WindowTreeHostMus> window_tree_host) final;
+  void OnEmbedRootReady(aura::WindowTreeHostMus* window_tree_host) final;
   void OnEmbedRootDestroyed(aura::WindowTreeHostMus* window_tree_host) final;
 
-  size_t initialized_windows_count_ = 0;
-
   size_t number_of_windows_ = 1;
 
   DISALLOW_COPY_AND_ASSIGN(MusDemoExternal);