Introduce isHTMLOptGroupElement and toHTMLOptGroupElement

Source/core/html/HTMLSelectElement.cpp

Index: Source/core/html/HTMLSelectElement.cpp
diff --git a/Source/core/html/HTMLSelectElement.cpp b/Source/core/html/HTMLSelectElement.cpp
index f5f2bdf9f077e20402ed36a4af5ef94e01b858ad..53e16a6e76b25de86cff2ad0e40df0206e17c90c 100644
--- a/Source/core/html/HTMLSelectElement.cpp
+++ b/Source/core/html/HTMLSelectElement.cpp
@@ -40,6 +40,7 @@
 #include "core/html/FormController.h"
 #include "core/html/FormDataList.h"
 #include "core/html/HTMLFormElement.h"
+#include "core/html/HTMLOptGroupElement.h"
 #include "core/html/HTMLOptionElement.h"
 #include "core/html/HTMLOptionsCollection.h"
 #include "core/page/EventHandler.h"
@@ -339,7 +340,7 @@ bool HTMLSelectElement::childShouldCreateRenderer(const NodeRenderingContext& ch
     if (!HTMLFormControlElementWithState::childShouldCreateRenderer(childContext))
         return false;
     if (!usesMenuList())
-        return childContext.node()->hasTagName(HTMLNames::optionTag) || childContext.node()->hasTagName(HTMLNames::optgroupTag);
+        return childContext.node()->hasTagName(HTMLNames::optionTag) || isHTMLOptGroupElement(childContext.node());
     return false;
 }
 
@@ -745,7 +746,7 @@ void HTMLSelectElement::recalcListItems(bool updateSelectedStates) const
         // optgroup tags may not nest. However, both FireFox and IE will
         // flatten the tree automatically, so we follow suit.
         // (http://www.w3.org/TR/html401/interact/forms.html#h-17.6)
-        if (current->hasTagName(optgroupTag)) {
+        if (isHTMLOptGroupElement(current)) {
             m_listItems.append(current);
             if (Element* nextElement = ElementTraversal::firstWithin(current)) {
                 currentElement = nextElement;