MD-Settings: convert cr-settings-prefs to a singleton model

chrome/browser/resources/settings/pref_tracker/pref_tracker.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
  * `cr-settings-pref-tracker` is a utility element used to track the
  * initialization of a specified preference and throw an error if the pref
  * is not defined after prefs have all been fetched.
  *
  * Example:
  *
  *    <cr-settings-pref-tracker pref="{{prefs.settings.foo.bar}}">
  *    </cr-settings-pref-tracker>
  *
  * @element cr-settings-pref-tracker
  */
 (function() {
 
-  /**
-   * An array of all the tracker instances.
-   * @type {!Array<!CrSettingsPrefTrackerElement>}
-   */
-  var instances = [];
-
-  /**
-   * Validates all tracker instances.
-   * @private
-   */
-  var validateAll_ = function() {
-    instances.forEach(function(tracker) {
-      tracker.validate_();
-    });
-  };
-
-  document.addEventListener(CrSettingsPrefs.INITIALIZED, validateAll_);
-
   Polymer({
     is: 'cr-settings-pref-tracker',
 
     properties: {
       /**
        * The Preference object being tracked.
        * @type {?PrefObject}
        */
       pref: {
         type: Object,
         observer: 'validate_',
       },
     },
 
     /** @override */
     ready: function() {
       this.validate_();
-
-      instances.push(this);
     },
 
     /**
-     * Throws an error if prefs are initialized and the tracked pref is not
-     * found.
+     * Logs an error once prefs are initialized if the tracked pref is
+     * not found.
      * @private
      */
     validate_: function() {
-      this.async(function() {
+      CrSettingsPrefs.initialized.then(function() {
         // Note that null == undefined.
-        if (CrSettingsPrefs.isInitialized && this.pref == null) {
+        if (this.pref == null) {
           // HACK ALERT: This is the best clue we have as to the pref key for
           // this tracker. This value should not be relied upon anywhere or
           // actually used besides for this error message.
-          var parentControlHTML = this.parentNode && this.parentNode.host &&
-              this.parentNode.host.outerHTML;
+          var parentControlHTML = this.domHost && this.domHost.outerHTML;
 
-          throw new Error('Pref not found. Parent control:' +
+          console.error('Pref not found. Parent control:' +
               (parentControlHTML || 'Unknown'));
         }
-      });
+      }.bind(this));
     },
   });
 })();