code: function preferencesInitialize() {
      return this.initializedPromise =
          this._readFromStorage(DEFAULT_PREFERENCES).then(function(prefObj) {
        this.isInitializedPromiseResolved = true;
        if (prefObj) {
          this.prefs = prefObj;
        }
      }.bind(this));
    }
docs: Initialize and fetch the current preference values from storage.
  @return {Promise} A promise that is resolved when the preferences
  have been initialized.