Add toHTMLSummaryElement(), and use it.

Source/core/html/HTMLSummaryElement.h

Index: Source/core/html/HTMLSummaryElement.h
diff --git a/Source/core/html/HTMLSummaryElement.h b/Source/core/html/HTMLSummaryElement.h
index 67a1fe1d91144ea6cdaa58bcdd08482603c32456..01316113eb48e8c31e30ead681d3c9e6d041df6d 100644
--- a/Source/core/html/HTMLSummaryElement.h
+++ b/Source/core/html/HTMLSummaryElement.h
@@ -44,6 +44,12 @@ private:
     bool supportsFocus() const OVERRIDE;
 };
 
+inline HTMLSummaryElement* toHTMLSummaryElement(Node* node)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!node || node->hasTagName(HTMLNames::summaryTag));
+    return static_cast<HTMLSummaryElement*>(node);
+}
+
 }
 
 #endif // HTMLSummaryElement_h