Turn DocumentParser::pinToMainThread into a cleaner api

Source/core/xmlhttprequest/XMLHttpRequest.cpp

Index: Source/core/xmlhttprequest/XMLHttpRequest.cpp
diff --git a/Source/core/xmlhttprequest/XMLHttpRequest.cpp b/Source/core/xmlhttprequest/XMLHttpRequest.cpp
index d656155b3fdb1186e4d61032eada2a8d6afb07c3..6aacb15037412aa01f5786680dd6a72234bc41d3 100644
--- a/Source/core/xmlhttprequest/XMLHttpRequest.cpp
+++ b/Source/core/xmlhttprequest/XMLHttpRequest.cpp
@@ -1610,7 +1610,7 @@ void XMLHttpRequest::parseDocumentChunk(const char* data, unsigned len)
         if (!m_responseDocument)
             return;
 
-        m_responseDocumentParser = m_responseDocument->implicitOpen();
+        m_responseDocumentParser = m_responseDocument->implicitOpen(AllowAsynchronousParsing);
         m_responseDocumentParser->addClient(this);
     }
     ASSERT(m_responseDocumentParser);