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

chrome/browser/resources/settings/prefs/prefs.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-prefs' exposes a singleton model of Chrome settings and
  * preferences, which listens to changes to Chrome prefs whitelisted in
  * chrome.settingsPrivate. When changing prefs in this element's 'prefs'
  * property via the UI, the singleton model tries to set those preferences in
@@ skipping 263 matching lines @@
      * 'search.suggest_enabled'.
      * @param {string} path
      * @return {string}
      * @private
      */
     getPrefKeyFromPath_: function(path) {
       // Skip the first token, which refers to the member variable (this.prefs).
       var parts = path.split('.');
       assert(parts.shift() == 'prefs', "Path doesn't begin with 'prefs'");
 
-      for (let i = 1; i <= parts.length; i++) {
-        let key = parts.slice(0, i).join('.');
+      for (var i = 1; i <= parts.length; i++) {
+        var key = parts.slice(0, i).join('.');
         // The lastPrefValues_ keys match the pref keys.
         if (this.lastPrefValues_.hasOwnProperty(key))
           return key;
       }
       return '';
     },
 
     /**
      * Resets the element so it can be re-initialized with a new prefs state.
      */
     resetForTesting: function() {
       if (!this.initialized_)
         return;
       this.prefs = undefined;
       this.lastPrefValues_ = {};
       this.initialized_ = false;
       // Remove the listener added in initialize().
       this.settingsApi_.onPrefsChanged.removeListener(this.boundPrefsChanged_);
       this.settingsApi_ =
           /** @type {SettingsPrivate} */(chrome.settingsPrivate);
     },
   });
 })();