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

ash/mus/native_widget_factory_mus.h

+// Copyright 2016 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 ASH_MUS_NATIVE_WIDGET_FACTORY_MUS_H_
+#define ASH_MUS_NATIVE_WIDGET_FACTORY_MUS_H_
+
+#include "base/macros.h"
+
+namespace ash {
+namespace mus {
+
+class WindowManager;
+
+// A native widget factory used for widgets in the ash system UI itself.
+// For example, this creates context menu widgets for the shelf and wallpaper.
+class NativeWidgetFactoryMus {
+ public:
+  explicit NativeWidgetFactoryMus(WindowManager* window_manager);
+  ~NativeWidgetFactoryMus();
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(NativeWidgetFactoryMus);
+};
+
+}  // namespace mus
+}  // namespace ash
+
+#endif  // ASH_MUS_NATIVE_WIDGET_FACTORY_MUS_H_