Use Traversal<>::firstChild() instead of firstWithin() when iterating over children

Source/core/dom/Document.cpp

Index: Source/core/dom/Document.cpp
diff --git a/Source/core/dom/Document.cpp b/Source/core/dom/Document.cpp
index 996a28e911bf13003b2f50d01cf4cf77b99a505b..e0824fc280a9d3c2bff7ab28cef6bdde4f8de3bd 100644
--- a/Source/core/dom/Document.cpp
+++ b/Source/core/dom/Document.cpp
@@ -2398,7 +2398,7 @@ HTMLElement* Document::body() const
     if (!documentElement())
         return 0;
 
-    for (HTMLElement* child = Traversal<HTMLElement>::firstWithin(*documentElement()); child; child = Traversal<HTMLElement>::nextSibling(*child)) {
+    for (HTMLElement* child = Traversal<HTMLElement>::firstChild(*documentElement()); child; child = Traversal<HTMLElement>::nextSibling(*child)) {
         if (isHTMLFrameSetElement(*child) || isHTMLBodyElement(*child))
             return child;
     }