[ParseHTMLOnMainThread] Force plaintext synchronously on lookahead parser

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 ed5d3592f20c47ae3ecd43aa377039fe2f1c5f45..20960c46ec5a2edb8915bdbc199616621f286455 100644
--- a/third_party/WebKit/Source/core/html/parser/HTMLDocumentParser.cpp
+++ b/third_party/WebKit/Source/core/html/parser/HTMLDocumentParser.cpp
@@ -573,7 +573,9 @@ void HTMLDocumentParser::forcePlaintextForTextDocument()
         if (!m_haveBackgroundParser)
             startBackgroundParser();
 
-        postTaskToLookaheadParser(Asynchronous, &BackgroundHTMLParser::forcePlaintextForTextDocument, m_backgroundParser);
+        // This task should be synchronous, because otherwise synchronous
+        // tokenizing can happen before plaintext is forced.
+        postTaskToLookaheadParser(Synchronous, &BackgroundHTMLParser::forcePlaintextForTextDocument, m_backgroundParser);
     } else
         m_tokenizer->setState(HTMLTokenizer::PLAINTEXTState);
 }