Mus Demo: Refactor code to prepare multiple windows in external mode

services/ui/demo/window_tree_data.h

Index: services/ui/demo/window_tree_data.h
diff --git a/services/ui/demo/window_tree_data.h b/services/ui/demo/window_tree_data.h
index 0e1bcbd0806f8cc96f1feb5cdaf4c26a18b5d571..007b04f6d284c4d87c9775f239c2a8c65562e9c3 100644
--- a/services/ui/demo/window_tree_data.h
+++ b/services/ui/demo/window_tree_data.h
@@ -29,6 +29,10 @@ class WindowTreeData {
   void Init(std::unique_ptr<aura::WindowTreeHostMus> window_tree_host);
   bool IsInitialized() const { return !!window_tree_host_; }
 
+  const aura::WindowTreeHostMus* WindowTreeHost() const {
+    return window_tree_host_.get();
+  }
+
  private:
   // Draws one frame, incrementing the rotation angle.
   void DrawFrame();