Consistently use ElementTraversal API when looking for an Element

Source/core/html/HTMLDetailsElement.cpp

Index: Source/core/html/HTMLDetailsElement.cpp
diff --git a/Source/core/html/HTMLDetailsElement.cpp b/Source/core/html/HTMLDetailsElement.cpp
index 9546f306e698da65e31e050950e79d44ff043426..b08a2536e291e39b4924ff60df90f884573b97c1 100644
--- a/Source/core/html/HTMLDetailsElement.cpp
+++ b/Source/core/html/HTMLDetailsElement.cpp
@@ -79,9 +79,9 @@ void HTMLDetailsElement::didAddUserAgentShadowRoot(ShadowRoot& root)
 
 Element* HTMLDetailsElement::findMainSummary() const
 {
-    for (Node* child = firstChild(); child; child = child->nextSibling()) {
+    for (Element* child = ElementTraversal::firstWithin(*this); child; child = ElementTraversal::nextSibling(*child)) {
         if (child->hasTagName(summaryTag))
-            return toElement(child);
+            return child;
     }
 
     HTMLContentElement* content = toHTMLContentElement(userAgentShadowRoot()->firstChild());