MD Settings: Convert all usages of .bind(this) to use ES6 arrow function.

chrome/browser/resources/settings/controls/pref_control_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.
 
 /**
  * @polymerBehavior Tracks the initialization of a specified preference and
  * logs an error if the pref is not defined after prefs have been fetched.
  */
 var PrefControlBehavior = {
   properties: {
@@ skipping 11 matching lines @@
   /** @override */
   ready: function() {
     this.validatePref_();
   },
 
   /**
    * Logs an error once prefs are initialized if the tracked pref is not found.
    * @private
    */
   validatePref_: function() {
-    CrSettingsPrefs.initialized.then(function() {
+    CrSettingsPrefs.initialized.then(() => {
       if (!this.pref) {
         var error = 'Pref not found for element ' + this.tagName;
         if (this.id)
           error += '#' + this.id;
         error += ' in ' + this.domHost.tagName;
         console.error(error);
       }
-    }.bind(this));
+    });
   },
 };