MD Settings: Add Learn More button to the Passwords subpage

chrome/browser/resources/settings/settings_page/settings_subpage.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.
 
 /**
  * @fileoverview
  * 'settings-subpage' shows a subpage beneath a subheader. The header contains
  * the subpage title, a search field and a back icon.
  */
 
 Polymer({
   is: 'settings-subpage',
 
   behaviors: [
     // TODO(michaelpg): phase out NeonAnimatableBehavior.
     Polymer.NeonAnimatableBehavior,
     Polymer.IronResizableBehavior,
   ],
 
   properties: {
     pageTitle: String,
 
+    learnMoreUrl: String,
+
     /** Setting a |searchLabel| will enable search. */
     searchLabel: String,
 
     searchTerm: {
       type: String,
       notify: true,
       value: '',
     },
 
     /** If true shows an active spinner at the end of the subpage header. */
@@ skipping 17 matching lines @@
   /** @private */
   onTapBack_: function() {
     settings.navigateToPreviousRoute();
   },
 
   /** @private */
   onSearchChanged_: function(e) {
     this.searchTerm = e.detail;
   },
 });