Turn DocumentParser::pinToMainThread into a cleaner api

Source/core/html/parser/ParserSynchronizationPolicy.h

Index: Source/core/html/parser/ParserSynchronizationPolicy.h
diff --git a/Source/core/html/parser/ParserSynchronizationPolicy.h b/Source/core/html/parser/ParserSynchronizationPolicy.h
new file mode 100644
index 0000000000000000000000000000000000000000..6d93535626a13aa90feeb3eede8084f9c2333cbe
--- /dev/null
+++ b/Source/core/html/parser/ParserSynchronizationPolicy.h
@@ -0,0 +1,13 @@
+#ifndef ParserSynchronizationPolicy_h
+#define ParserSynchronizationPolicy_h
+
+namespace blink {
+
+enum ParserSynchronizationPolicy {
+    AllowAsynchronousParsing,
+    ForceSynchronousParsing,
+};
+
+}
+
+#endif // ParserSynchronizationPolicy_h