Turn DocumentParser::pinToMainThread into a cleaner api

Source/core/html/imports/HTMLImportLoader.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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 98 matching lines @@
 
     setState(finishWriting());
 }
 
 HTMLImportLoader::State HTMLImportLoader::startWritingAndParsing(const ResourceResponse& response)
 {
     ASSERT(!m_imports.isEmpty());
     DocumentInit init = DocumentInit(response.url(), 0, m_controller->master()->contextDocument(), m_controller)
         .withRegistrationContext(m_controller->master()->registrationContext());
     m_document = HTMLDocument::create(init);
-    m_writer = DocumentWriter::create(m_document.get(), response.mimeType(), "UTF-8");
+    m_writer = DocumentWriter::create(m_document.get(), AllowAsynchronousParsing, response.mimeType(), "UTF-8");
 
     DocumentParser* parser = m_document->parser();
     ASSERT(parser);
     parser->addClient(this);
 
     return StateLoading;
 }
 
 HTMLImportLoader::State HTMLImportLoader::finishWriting()
 {
@@ skipping 101 matching lines @@
 #if ENABLE(OILPAN)
     visitor->trace(m_imports);
 #endif
     visitor->trace(m_document);
     visitor->trace(m_writer);
     visitor->trace(m_microtaskQueue);
     DocumentParserClient::trace(visitor);
 }
 
 } // namespace blink