delete useThreading flag as it is always true

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 21 matching lines @@
 #include "core/frame/Settings.h"
 #include "core/loader/FrameLoader.h"
 
 namespace blink {
 
 HTMLParserOptions::HTMLParserOptions(Document* document)
 {
     LocalFrame* frame = document ? document->frame() : 0;
     scriptEnabled = frame && frame->script().canExecuteScripts(NotAboutToExecuteScript);
     pluginsEnabled = frame && frame->loader().allowPlugins(NotAboutToInstantiatePlugin);
-    useThreading = true;
 }
 
 }