Compile even more Files app targets in gyp v2.

ui/file_manager/externs/menu_item_update_event.js

Index: ui/file_manager/externs/menu_item_update_event.js
diff --git a/ui/file_manager/externs/menu_item_update_event.js b/ui/file_manager/externs/menu_item_update_event.js
new file mode 100644
index 0000000000000000000000000000000000000000..0666a9abf0e82dba0335fe3a22acabcf31d1a557
--- /dev/null
+++ b/ui/file_manager/externs/menu_item_update_event.js
@@ -0,0 +1,14 @@
+// Copyright 2017 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.
+
+/**
+ * @constructor
+ * @extends {Event}
+ */
+var MenuItemUpdateEvent = function() {};
+
+/**
+ * @type {!cr.ui.MenuButton}
+ */
+MenuItemUpdateEvent.prototype.menuButton;