Have ElementTraversal::firstWithin() take a reference

Source/core/dom/TreeScope.cpp

Index: Source/core/dom/TreeScope.cpp
diff --git a/Source/core/dom/TreeScope.cpp b/Source/core/dom/TreeScope.cpp
index 370397441482bf28bab149239580bf38e6c8bea7..1ec312f6a338e3a89c26d8acf020f7a08d812c11 100644
--- a/Source/core/dom/TreeScope.cpp
+++ b/Source/core/dom/TreeScope.cpp
@@ -277,7 +277,8 @@ HTMLLabelElement* TreeScope::labelElementForId(const AtomicString& forAttributeV
     if (!m_labelsByForAttribute) {
         // Populate the map on first access.
         m_labelsByForAttribute = adoptPtr(new DocumentOrderedMap);
-        for (Element* element = ElementTraversal::firstWithin(rootNode()); element; element = ElementTraversal::next(*element)) {
+        ASSERT(rootNode());
+        for (Element* element = ElementTraversal::firstWithin(*rootNode()); element; element = ElementTraversal::next(*element)) {
             if (isHTMLLabelElement(element)) {
                 HTMLLabelElement* label = toHTMLLabelElement(element);
                 const AtomicString& forValue = label->fastGetAttribute(forAttr);
@@ -311,7 +312,8 @@ Element* TreeScope::findAnchor(const String& name)
         return 0;
     if (Element* element = getElementById(name))
         return element;
-    for (Element* element = ElementTraversal::firstWithin(rootNode()); element; element = ElementTraversal::next(*element)) {
+    ASSERT(rootNode());
+    for (Element* element = ElementTraversal::firstWithin(*rootNode()); element; element = ElementTraversal::next(*element)) {
         if (isHTMLAnchorElement(element)) {
             HTMLAnchorElement* anchor = toHTMLAnchorElement(element);
             if (rootNode()->document().inQuirksMode()) {
@@ -485,7 +487,8 @@ Element* TreeScope::getElementByAccessKey(const String& key) const
         return 0;
     Element* result = 0;
     Node* root = rootNode();
-    for (Element* element = ElementTraversal::firstWithin(root); element; element = ElementTraversal::next(*element, root)) {
+    ASSERT(root);
+    for (Element* element = ElementTraversal::firstWithin(*root); element; element = ElementTraversal::next(*element, root)) {
         if (equalIgnoringCase(element->fastGetAttribute(accesskeyAttr), key))
             result = element;
         for (ShadowRoot* shadowRoot = element->youngestShadowRoot(); shadowRoot; shadowRoot = shadowRoot->olderShadowRoot()) {