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

chrome/browser/resources/settings/controls/settings_dropdown_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.
 
 /**
  * The |name| is shown in the gui.  The |value| us use to set or compare with
  * the preference value.
  * @typedef {{
  *   name: string,
  *   value: (number|string)
@@ skipping 79 matching lines @@
     if (this.menuOptions === null || !this.menuOptions.length)
       return;
 
     var prefValue = this.pref.value;
     var option = this.menuOptions.find(function(menuItem) {
       return menuItem.value == prefValue;
     });
 
     // Wait for the dom-repeat to populate the <select> before setting
     // <select>#value so the correct option gets selected.
-    this.async(function() {
+    this.async(() => {
       this.$.dropdownMenu.value = option == undefined ?
           this.notFoundValue_ :
           Settings.PrefUtil.prefToString(assert(this.pref));
-    }.bind(this));
+    });
   },
 
   /**
    * @param {?DropdownMenuOptionList} menuOptions
    * @param {string} prefValue
    * @return {boolean}
    * @private
    */
   showNotFoundValue_: function(menuOptions, prefValue) {
     // Don't show "Custom" before the options load.
     if (!menuOptions || !menuOptions.length)
       return false;
 
     var option = menuOptions.find(function(menuItem) {
       return menuItem.value == prefValue;
     });
     return !option;
   },
 
   /**
    * @return {boolean}
    * @private
    */
   shouldDisableMenu_: function() {
     return this.disabled || this.isPrefEnforced() ||
         this.menuOptions === null || this.menuOptions.length == 0;
   },
 });