Introduce isHTMLTableElement and toHTMLTableElement

Source/core/html/HTMLTablePartElement.cpp

Index: Source/core/html/HTMLTablePartElement.cpp
diff --git a/Source/core/html/HTMLTablePartElement.cpp b/Source/core/html/HTMLTablePartElement.cpp
index 25644e7413fb0c2a0b0be9d17a7cc6d984616a3e..2373e5291e619ae72cc9cb923435b94770de594e 100644
--- a/Source/core/html/HTMLTablePartElement.cpp
+++ b/Source/core/html/HTMLTablePartElement.cpp
@@ -86,9 +86,9 @@ void HTMLTablePartElement::collectStyleForPresentationAttribute(const QualifiedN
 HTMLTableElement* HTMLTablePartElement::findParentTable() const
 {
     ContainerNode* parent = NodeRenderingTraversal::parent(this);
-    while (parent && !parent->hasTagName(tableTag))
+    while (parent && !isHTMLTableElement(parent))
         parent = NodeRenderingTraversal::parent(parent);
-    return static_cast<HTMLTableElement*>(parent);
+    return toHTMLTableElement(parent);
 }
 
 }