gtk: Add dummy implementation of Menu.

browser/api/atom_api_menu_gtk.h

@@ -0,0 +1,30 @@
+// Copyright (c) 2014 GitHub, Inc. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef ATOM_BROWSER_API_ATOM_API_MENU_GTK_H_
+#define ATOM_BROWSER_API_ATOM_API_MENU_GTK_H_
+
+#include "browser/api/atom_api_menu.h"
+
+namespace atom {
+
+namespace api {
+
+class MenuGtk : public Menu {
+ public:
+  explicit MenuGtk(v8::Handle<v8::Object> wrapper);
+  virtual ~MenuGtk();
+
+ protected:
+  virtual void Popup(NativeWindow* window) OVERRIDE;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(MenuGtk);
+};
+
+}  // namespace api
+
+}  // namespace atom
+
+#endif  // ATOM_BROWSER_API_ATOM_API_MENU_GTK_H_