[Closure compiler] Fix extension_error_overlay.js closure compiler

chrome/browser/resources/extensions/extension_code.js

 // Copyright 2014 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.
 
 /**
  * @typedef {{afterHighlight: string,
  *            beforeHighlight: string,
  *            highlight: string,
  *            title: string}}
  */
@@ skipping 12 matching lines @@
     div.__proto__ = ExtensionCode.prototype;
     return div;
   }
 
   ExtensionCode.prototype = {
     __proto__: HTMLDivElement.prototype,
 
     /**
      * Populate the content area of the code div with the given code. This will
      * highlight the erroneous section (if any).
-     * @param {ExtensionHighlight} code The 'highlight' strings represent the
+     * @param {?ExtensionHighlight} code The 'highlight' strings represent the

[Devlin, 2015/03/09 17:35:51]

This fixes the compilation error, but why is it needed? 
https://developers.google.com/closure/compiler/docs/js-for-compiler states that
"All object types are nullable by default whether or not they are declared with
the Nullable operator".

      *     three portions of the file's content to display - the portion which
      *     is most relevant and should be emphasized (highlight), and the parts
      *     both before and after this portion. The title is the error message,
      *     which will be the mouseover hint for the highlighted region. These
      *     may be empty.
      *  @param {string} emptyMessage The message to display if the code
      *     object is empty (e.g., 'could not load code').
      */
     populate: function(code, emptyMessage) {
       // Clear any remnant content, so we don't have multiple code listed.
@@ skipping 65 matching lines @@
       if (errorSpan)
         this.scrollTop = errorSpan.offsetTop - this.clientHeight / 2;
     }
   };
 
   // Export
   return {
     ExtensionCode: ExtensionCode
   };
 });