Make XHR use the background HTML parser

Source/core/html/parser/HTMLParserOptions.cpp

 /*
  * Copyright (C) 2013 Google, Inc. All Rights Reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 31 matching lines @@
 
     // We force the main-thread parser for two cases:
     // - about:blank and javascript (which uses about:blank) for compatibility
     //   with historical synchronous loading/parsing behavior.
     // - instances where the Document has no Frame (this happens sometimes for
     //   HTML imports, and possibly other cases).
     // FIXME: We want to use the threaded parser for XHRs (where there is no
     // frame) so the second case should go away eventually.
     // FIXME: Gecko does not load javascript: urls synchronously, why do we?
     // See LayoutTests/loader/iframe-sync-loads.html
-    useThreading = document && document->frame() && !document->url().isAboutBlankURL();
+    useThreading = document && !document->url().isAboutBlankURL();
 }
 
 }