[Closure compiler] Fix extension_error_overlay.js closure compiler

chrome/browser/resources/extensions/extension_code.js

Index: chrome/browser/resources/extensions/extension_code.js
diff --git a/chrome/browser/resources/extensions/extension_code.js b/chrome/browser/resources/extensions/extension_code.js
index 826625422ceefaf47ec36a1d8964f7bcbeb5a2a6..1e95c88d9ea7ac19a7c8317ddccb83171086015c 100644
--- a/chrome/browser/resources/extensions/extension_code.js
+++ b/chrome/browser/resources/extensions/extension_code.js
@@ -30,7 +30,7 @@ cr.define('extensions', function() {
     /**
      * 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,