Have LiveNodeList::rootNode() return a reference

Source/core/dom/LiveNodeList.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2006, 2007 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
@@ skipping 82 matching lines @@
         else if (hasIdNameCache() && (*attrName == HTMLNames::idAttr || *attrName == HTMLNames::nameAttr))
             invalidateIdNameCacheMaps();
     }
     void invalidateCache() const;
     void invalidateIdNameCacheMaps() const;
 
     static bool shouldInvalidateTypeOnAttributeChange(NodeListInvalidationType, const QualifiedName&);
 
 protected:
     Document& document() const { return m_ownerNode->document(); }
-    Node* rootNode() const;
+    Node& rootNode() const;
     ContainerNode* rootContainerNode() const;
     bool overridesItemAfter() const { return m_overridesItemAfter; }
 
     ALWAYS_INLINE bool isItemCacheValid() const { return m_isItemCacheValid; }
     ALWAYS_INLINE Node* cachedItem() const { return m_cachedItem; }
     ALWAYS_INLINE unsigned cachedItemOffset() const { return m_cachedItemOffset; }
 
     ALWAYS_INLINE bool isLengthCacheValid() const { return m_isLengthCacheValid; }
     ALWAYS_INLINE unsigned cachedLength() const { return m_cachedLength; }
     ALWAYS_INLINE void setLengthCache(unsigned length) const
@@ skipping 82 matching lines @@
     virtual Node* namedItem(const AtomicString&) const OVERRIDE;
     virtual bool nodeMatches(Element*) const = 0;
 
 private:
     virtual bool isLiveNodeList() const OVERRIDE { return true; }
 };
 
 } // namespace WebCore
 
 #endif // LiveNodeList_h