Fix Closure compilation error in I18nBehavior.

ui/webui/resources/js/i18n_behavior.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
  * 'I18nBehavior' is a behavior to mix in loading of
  * internationalization strings.
  *
  * Example:
  *   behaviors: [
  *     I18nBehavior,
  *   ],
  */
 
 /** @polymerBehavior */
 var I18nBehavior = {
   /**
    * @param {string} id The ID of the string to translate.
-   * @param {...string=} var_args Placeholders required by the string ($0-9).
+   * @param {...string} var_args Placeholders required by the string ($0-9).
    * @return {string} A translated, substituted string.
    */
   i18n: function(id, var_args) {
     return arguments.length == 1 ? loadTimeData.getString(id) :
         loadTimeData.getStringF.apply(loadTimeData, arguments);
   },
 };