Switch MD Settings to paper-dropdown-menu-light

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 78 matching lines @@
 
   /**
    * Check to see if we have all the pieces needed to enable the control.
    * @param {DropdownMenuOptionList} menuOptions
    * @private
    */
   checkSetup_: function(menuOptions) {
     if (!this.menuOptions.length)
       return;
 
-    this.menuLabel_ = this.label;
+    // Do not set |menuLabel_| to a falsy value: http://goo.gl/OnKYko
+    // (paper-dropdown-menu#181).
+    this.menuLabel_ = this.label || ' ';
     this.updateSelected_();
   },
 
   /**
    * Pass the selection change to the pref value.
    * @private
    */
   onSelect_: function() {
     if (!this.pref || this.selected_ == undefined ||
         this.selected_ == this.notFoundValue_) {
@@ skipping 32 matching lines @@
   },
 
   /**
    * @return {boolean}
    * @private
    */
   shouldDisableMenu_: function() {
     return this.disabled || !this.menuOptions.length;
   },
 });