WebUI: Undo some usages of ES6 features that break uglify.

chrome/browser/resources/settings/settings_page/main_page_behavior.js

 // 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.
 
 /**
  * Responds to route changes by expanding, collapsing, or scrolling to sections
  * on the page. Expanded sections take up the full height of the container. At
  * most one section should be expanded at any given time.
  * @polymerBehavior MainPageBehavior
  */
@@ skipping 271 matching lines @@
   /**
   /**
    * Hides or unhides the sections not being expanded.
    * @param {string} sectionName The section to keep visible.
    * @param {boolean} hidden Whether the sections should be hidden.
    * @private
    */
   toggleOtherSectionsHidden_: function(sectionName, hidden) {
     var sections = Polymer.dom(this.root).querySelectorAll(
         'settings-section');
-    for (var section of sections)
-      section.hidden = hidden && (section.section != sectionName);
+    for (var i = 0; i < sections.length; i++)
+      sections[i].hidden = hidden && (sections[i].section != sectionName);
   },
 
   /**
    * Helper function to get a section from the local DOM.
    * @param {string} section Section name of the element to get.
    * @return {?SettingsSectionElement}
    */
   getSection: function(section) {
     if (!section)
       return null;
     return /** @type {?SettingsSectionElement} */(
         this.$$('settings-section[section="' + section + '"]'));
   },
 };
 
 /** @polymerBehavior */
 var MainPageBehavior = [
   settings.RouteObserverBehavior,
   MainPageBehaviorImpl,
 ];