MD Settings: use custom iconset

chrome/browser/resources/settings/settings_menu/settings_menu.js

 // 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
  * 'settings-menu' shows a menu with a hardcoded set of pages and subpages.
  *
  * Example:
  *
@@ skipping 39 matching lines @@
       };
     }
   },
 
   /**
    * @param {boolean} opened Whether the menu is expanded.
    * @return {string} Which icon to use.
    * @private
    * */
   arrowState_: function(opened) {
-    return opened ? 'arrow-drop-up' : 'arrow-drop-down';
+    return opened ? 'settings:arrow-drop-up' : 'settings:arrow-drop-down';
   },
 });