Add toHTMLFieldSetElement and toHTMLLegendElement, and use them.

Source/core/html/HTMLFieldSetElement.cpp

Index: Source/core/html/HTMLFieldSetElement.cpp
diff --git a/Source/core/html/HTMLFieldSetElement.cpp b/Source/core/html/HTMLFieldSetElement.cpp
index 12c54de735ed7a08e2e79826ee1b027f9596dd18..23ffe90e6358900def04f43f78c9b4fb7e18e5bc 100644
--- a/Source/core/html/HTMLFieldSetElement.cpp
+++ b/Source/core/html/HTMLFieldSetElement.cpp
@@ -94,7 +94,7 @@ HTMLLegendElement* HTMLFieldSetElement::legend() const
 {
     for (Element* child = ElementTraversal::firstWithin(this); child; child = ElementTraversal::nextSkippingChildren(child, this)) {
         if (child->hasTagName(legendTag))
-            return static_cast<HTMLLegendElement*>(child);
+            return toHTMLLegendElement(child);
     }
     return 0;
 }