Count usage of the obsolete 'isindex' element.

Source/core/html/parser/HTMLDocumentParser.h

Index: Source/core/html/parser/HTMLDocumentParser.h
diff --git a/Source/core/html/parser/HTMLDocumentParser.h b/Source/core/html/parser/HTMLDocumentParser.h
index b41beae929c1c532d848ad2f2096f35b4cfab52a..40779561ad36e7acbe3004ab4bcaa7410e8ba133 100644
--- a/Source/core/html/parser/HTMLDocumentParser.h
+++ b/Source/core/html/parser/HTMLDocumentParser.h
@@ -29,6 +29,7 @@
 #include "core/dom/ParserContentPolicy.h"
 #include "core/dom/ScriptableDocumentParser.h"
 #include "core/fetch/ResourceClient.h"
+#include "core/frame/UseCounter.h"
 #include "core/html/parser/BackgroundHTMLInputStream.h"
 #include "core/html/parser/CompactHTMLToken.h"
 #include "core/html/parser/HTMLInputStream.h"
@@ -96,6 +97,8 @@ public:
     };
     void didReceiveParsedChunkFromBackgroundParser(PassOwnPtr<ParsedChunk>);
 
+    UseCounter* useCounter() { return UseCounter::getFrom(contextForParsingSession()); }
+
 protected:
     virtual void insert(const SegmentedString&) OVERRIDE;
     virtual void append(PassRefPtr<StringImpl>) OVERRIDE;