Add toHTMLMarqueeElement, and use it.

Source/core/rendering/RenderMarquee.cpp

Index: Source/core/rendering/RenderMarquee.cpp
diff --git a/Source/core/rendering/RenderMarquee.cpp b/Source/core/rendering/RenderMarquee.cpp
index 9540f0b3a6c1464c36a76e110dba431ce2dbe549..84d18eae1b08da20c72d72e197d8014a9fbda2c3 100644
--- a/Source/core/rendering/RenderMarquee.cpp
+++ b/Source/core/rendering/RenderMarquee.cpp
@@ -82,11 +82,8 @@ RenderMarquee::~RenderMarquee()
 int RenderMarquee::marqueeSpeed() const
 {
     int result = style()->marqueeSpeed();
-    if (Node* node = this->node()) {
-        ASSERT(node->hasTagName(marqueeTag));
-        HTMLMarqueeElement* marqueeElt = static_cast<HTMLMarqueeElement*>(node);
-        result = max(result, marqueeElt->minimumDelay());
-    }
+    if (Node* node = this->node())
+        result = std::max(result, toHTMLMarqueeElement(node)->minimumDelay());
     return result;
 }