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

ash/mus/window_manager.h

Index: ash/mus/window_manager.h
diff --git a/ash/mus/window_manager.h b/ash/mus/window_manager.h
index 67ea7ca4d048a0f56241c4f3e56616ca6825be8d..4b78ed011654bc159f1cf6354ce1518154a14e06 100644
--- a/ash/mus/window_manager.h
+++ b/ash/mus/window_manager.h
@@ -35,6 +35,7 @@ namespace ash {
 namespace mus {
 
 class AcceleratorHandler;
+class NativeWidgetFactoryMus;
 class RootWindowController;
 class ShadowController;
 class WindowManagerObserver;
@@ -127,6 +128,8 @@ class WindowManager : public ui::WindowManagerDelegate,
 
   ui::WindowManagerClient* window_manager_client_ = nullptr;
 
+  std::unique_ptr<NativeWidgetFactoryMus> native_widget_factory_mus_;
+
   std::unique_ptr<views::PointerWatcherEventRouter>
       pointer_watcher_event_router_;