Turn DocumentParser::pinToMainThread into a cleaner api

Source/core/dom/Document.h

Index: Source/core/dom/Document.h
diff --git a/Source/core/dom/Document.h b/Source/core/dom/Document.h
index b42c576617870d330c5e9bbd4bb249835a354a63..d8ea48e8255db9a463219b0ee309294a917b36c9 100644
--- a/Source/core/dom/Document.h
+++ b/Source/core/dom/Document.h
@@ -47,6 +47,7 @@
 #include "core/dom/custom/CustomElement.h"
 #include "core/frame/LocalDOMWindow.h"
 #include "core/html/CollectionType.h"
+#include "core/html/parser/ParserSynchronizationPolicy.h"
 #include "core/page/FocusType.h"
 #include "core/page/PageVisibilityState.h"
 #include "platform/Length.h"
@@ -511,6 +512,8 @@ public:
     void open(Document* ownerDocument = nullptr, ExceptionState& = ASSERT_NO_EXCEPTION);
     PassRefPtrWillBeRawPtr<DocumentParser> implicitOpen();
 
+    void forceSynchronousParsing();
+
     // close() is the DOM API document.close()
     void close(ExceptionState& = ASSERT_NO_EXCEPTION);
     // In some situations (see the code), we ignore document.close().
@@ -1382,6 +1385,8 @@ private:
     DocumentVisibilityObserverSet m_visibilityObservers;
 
     int m_styleRecalcElementCounter;
+
+    ParserSynchronizationPolicy m_backgroundParsingPolicy;
 };
 
 inline bool Document::shouldOverrideLegacyDescription(ViewportDescription::Type origin)