Mus Demo: Add a comment line switch for launching multiple windows in external mode

services/ui/demo/mus_demo_external.h

 // Copyright 2017 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 SERVICES_UI_DEMO_MUS_DEMO_EXTERNAL_H_
 #define SERVICES_UI_DEMO_MUS_DEMO_EXTERNAL_H_
 
 #include <memory>
 #include <vector>
 
@@ skipping 18 matching lines @@
   std::unique_ptr<aura::WindowTreeClient> CreateWindowTreeClient() final;
 
   // aura::WindowTreeClientDelegate:
   void OnEmbed(std::unique_ptr<aura::WindowTreeHostMus> window_tree_host) final;
   void OnEmbedRootDestroyed(aura::WindowTreeHostMus* window_tree_host) final;
 
   mojom::WindowTreeHostFactoryPtr window_tree_host_factory_;
   mojom::WindowTreeClientPtr window_tree_client_mojo_;
   size_t initialized_windows_count_ = 0;
 
+  size_t number_of_windows_ = 1;
+
   DISALLOW_COPY_AND_ASSIGN(MusDemoExternal);
 };
 
 }  // namespace demo
 }  // namespace ui
 
 #endif  // SERVICES_UI_DEMO_MUS_DEMO_EXTERNAL_H_