Add a <cr-dropdown-menu> custom element to cr_elements.

ui/webui/resources/cr_elements/cr_dropdown_menu/cr_dropdown_menu.js

Index: ui/webui/resources/cr_elements/cr_dropdown_menu/cr_dropdown_menu.js
diff --git a/ui/webui/resources/cr_elements/cr_dropdown_menu/cr_dropdown_menu.js b/ui/webui/resources/cr_elements/cr_dropdown_menu/cr_dropdown_menu.js
new file mode 100644
index 0000000000000000000000000000000000000000..6eeaa94d0bc03a15b60f85611c97cef9a91fb6db
--- /dev/null
+++ b/ui/webui/resources/cr_elements/cr_dropdown_menu/cr_dropdown_menu.js
@@ -0,0 +1,45 @@
+/* Copyright 2015 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. */
+
+/**
+ * @fileoverview
+ * 'cr-dropdown-menu' is a convenient wrapper around paper-dropdown-menu. It

[stevenjb, 2015/02/12 18:47:34]

We should probably standardize these comments. I would suggest something like
'is a chrome specific wrapper around'...

[Jeremy Klein, 2015/02/12 20:32:39]

Done.

+ * includes a paper-dropdown so its content should just be a core-menu and
+ * items.
+ *
+ * Example:
+ *   <cr-dropdown-menu>
+ *     <core-menu>
+ *       <paper-item>Chrome</paper-item>
+ *       <paper-item>Firefox</paper-item>
+ *       <paper-item>IE</paper-item>
+ *       <paper-item>Opera</paper-item>
+ *     </core-menu>
+ *   </cr-dropdown-menu>
+ *
+ * @group Chrome Elements
+ * @element cr-dropdown-menu
+ */
+
+Polymer('cr-dropdown-menu', {
+  publish: {
+    /**
+     * True if the menu is open.
+     *
+     * @attribute opened
+     * @type boolean
+     * @default false
+     */
+    opened: false,
+
+    /**
+     * A label for the control. The label is displayed if no item is selected.
+     *
+     * @attribute label
+     * @type string
+     * @default 'Select an item'
+     */
+    label: 'Select an item',

[Jeremy Klein, 2015/02/11 23:44:58]

Anyone have thoughts on i18n here?

[stevenjb, 2015/02/12 18:44:52]

The default label should probably be something that is clearly not intentional,
e.g. '<Dropdown Menu Label>'. That said, I think that eventually this element
(and other cr- elements) should take care of localization when 'label' is set
(and when the element is created), which suggests that perhaps we need a
separate 'translatedLabel' property also?

[Jeremy Klein, 2015/02/12 20:32:39]

Done.
+1, good point. Discussed in the dev meeting.

+  },
+});