Have Node::document() return a reference instead of a pointer

Source/core/dom/LiveNodeList.h

Index: Source/core/dom/LiveNodeList.h
diff --git a/Source/core/dom/LiveNodeList.h b/Source/core/dom/LiveNodeList.h
index 06e6d8f04399eb55fc23132162bcda807791eb3c..97dd515544a081d6c5657ec2bb06e095b5bef11e 100644
--- a/Source/core/dom/LiveNodeList.h
+++ b/Source/core/dom/LiveNodeList.h
@@ -68,13 +68,13 @@ public:
         ASSERT(!m_overridesItemAfter || !isNodeList(collectionType));
 
         if (collectionType != ChildNodeListType)
-            document()->registerNodeList(this);
+            document().registerNodeList(this);
     }
 
     virtual ~LiveNodeListBase()
     {
         if (type() != ChildNodeListType)
-            document()->unregisterNodeList(this);
+            document().unregisterNodeList(this);
     }
 
     // DOM API
@@ -99,7 +99,7 @@ public:
     static bool shouldInvalidateTypeOnAttributeChange(NodeListInvalidationType, const QualifiedName&);
 
 protected:
-    Document* document() const { return m_ownerNode->document(); }
+    Document& document() const { return m_ownerNode->document(); }
     Node* rootNode() const;
     ContainerNode* rootContainerNode() const;
     bool overridesItemAfter() const { return m_overridesItemAfter; }