Add toHTMLMarqueeElement, and use it.

Source/core/html/HTMLMarqueeElement.h

Index: Source/core/html/HTMLMarqueeElement.h
diff --git a/Source/core/html/HTMLMarqueeElement.h b/Source/core/html/HTMLMarqueeElement.h
index cfc4fb2c48e7f6b5f2d8052f3c282ce80d9d4e21..9169908586aa145b5fa953f2974c1141f3b12ebb 100644
--- a/Source/core/html/HTMLMarqueeElement.h
+++ b/Source/core/html/HTMLMarqueeElement.h
@@ -67,6 +67,12 @@ private:
     RenderMarquee* renderMarquee() const;
 };
 
+inline HTMLMarqueeElement* toHTMLMarqueeElement(Node* node)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!node || node->hasTagName(HTMLNames::marqueeTag));
+    return static_cast<HTMLMarqueeElement*>(node);
+}
+
 } // namespace WebCore
 
 #endif // HTMLMarqueeElement_h