mash: Port mojo:ash_sysui's ContextMenuMus to mojo:ash.

ash/mus/window_manager_application.cc

Index: ash/mus/window_manager_application.cc
diff --git a/ash/mus/window_manager_application.cc b/ash/mus/window_manager_application.cc
index 412b4e3fa41aa50de6cb9992a568469ef0a90339..809a2e56966ad60b772e6cf298e7ad020a3c7880 100644
--- a/ash/mus/window_manager_application.cc
+++ b/ash/mus/window_manager_application.cc
@@ -8,6 +8,7 @@
 
 #include "ash/common/material_design/material_design_controller.h"
 #include "ash/mus/accelerators/accelerator_registrar_impl.h"
+#include "ash/mus/native_widget_factory_mus.h"
 #include "ash/mus/window_manager.h"
 #include "base/bind.h"
 #include "base/memory/ptr_util.h"
@@ -110,6 +111,9 @@ void WindowManagerApplication::OnStart(const shell::Identity& identity) {
       window_manager_.get(), window_manager_.get(), nullptr);
   window_tree_client->ConnectAsWindowManager(connector());
 
+  native_widget_factory_mus_.reset(
+      new NativeWidgetFactoryMus(window_manager_.get()));
+
   InitWindowManager(window_tree_client);
 }