Count usage of the obsolete 'isindex' element.

Source/core/html/parser/HTMLTreeBuilder.cpp

Index: Source/core/html/parser/HTMLTreeBuilder.cpp
diff --git a/Source/core/html/parser/HTMLTreeBuilder.cpp b/Source/core/html/parser/HTMLTreeBuilder.cpp
index 77d6d24fc9225381237eb8f9d2d28d1f895baf61..4196cde09f0b85887fad3764755315671719ed5c 100644
--- a/Source/core/html/parser/HTMLTreeBuilder.cpp
+++ b/Source/core/html/parser/HTMLTreeBuilder.cpp
@@ -493,6 +493,10 @@ void HTMLTreeBuilder::processIsindexStartTagForInBody(AtomicHTMLToken* token)
 {
     ASSERT(token->type() == HTMLToken::StartTag);
     ASSERT(token->name() == isindexTag);
+
+    if (m_parser->useCounter())
+        m_parser->useCounter()->count(UseCounter::IsIndexElement);
+
     parseError(token);
     if (m_tree.form())
         return;