Allow parallel creation of windows

ui/aura/mus/window_tree_client_delegate.h

Index: ui/aura/mus/window_tree_client_delegate.h
diff --git a/ui/aura/mus/window_tree_client_delegate.h b/ui/aura/mus/window_tree_client_delegate.h
index e76d2efbac0b596f543f1e0765080ee391e24b38..12495bf2afb357acb61517421597a50ff8706ff1 100644
--- a/ui/aura/mus/window_tree_client_delegate.h
+++ b/ui/aura/mus/window_tree_client_delegate.h
@@ -27,6 +27,10 @@ class AURA_EXPORT WindowTreeClientDelegate {
   // InterfaceRequest.
   virtual void OnEmbed(std::unique_ptr<WindowTreeHostMus> window_tree_host) = 0;
 
+  // Called when the application implementing this interface is embedded and
+  // uses external window mode.
+  virtual void OnRootWindowEmbed(WindowTreeHostMus* window_tree_host);
+
   // Sent when another app is embedded in |root| (one of the roots of the
   // connection). Afer this call |root| is deleted. If the associated
   // WindowTreeClient was created from a WindowTreeClientRequest then