Add an experiment group via Settings to tokenize synchronously

third_party/WebKit/Source/core/html/parser/HTMLDocumentParser.cpp

Index: third_party/WebKit/Source/core/html/parser/HTMLDocumentParser.cpp
diff --git a/third_party/WebKit/Source/core/html/parser/HTMLDocumentParser.cpp b/third_party/WebKit/Source/core/html/parser/HTMLDocumentParser.cpp
index 61170c3f92f950b69419e723d5824542fba9798a..4d4ad321e2287bc9902cbee77211159338b7cf16 100644
--- a/third_party/WebKit/Source/core/html/parser/HTMLDocumentParser.cpp
+++ b/third_party/WebKit/Source/core/html/parser/HTMLDocumentParser.cpp
@@ -1025,7 +1025,8 @@ void HTMLDocumentParser::appendBytes(const char* data, size_t length)
         memcpy(buffer->data(), data, length);
         TRACE_EVENT1(TRACE_DISABLED_BY_DEFAULT("blink.debug"), "HTMLDocumentParser::appendBytes", "size", (unsigned)length);
 
-        postTaskToLookaheadParser(crossThreadBind(&BackgroundHTMLParser::appendRawBytesFromMainThread, m_backgroundParser, passed(std::move(buffer)), bytesReceivedTime));
+        LookaheadParserTaskSynchrony policy = document()->settings() && document()->settings()->parseHTMLOnMainThreadSyncTokenize() ? Synchronous : Asynchronous;
+        postTaskToLookaheadParser(crossThreadBind(&BackgroundHTMLParser::appendRawBytesFromMainThread, m_backgroundParser, passed(std::move(buffer)), bytesReceivedTime), policy);
         return;
     }