code: function preferencesReset() {
      return this.initializedPromise.then(function() {
        this.prefs = Object.create(DEFAULT_PREFERENCES);
        return this._writeToStorage(DEFAULT_PREFERENCES);
      }.bind(this));
    }
docs: Reset the preferences to their default values and update storage.
  @return {Promise} A promise that is resolved when the preference values
  have been reset.