Have Document::topDocument() return a reference

Source/core/accessibility/AXObjectCache.h

 /*
  * Copyright (C) 2003, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 61 matching lines @@
     };
 
     HashMap<AXID, CachedAXObjectAttributes> m_idMapping;
 };
 
 enum PostType { PostSynchronously, PostAsynchronously };
 
 class AXObjectCache {
     WTF_MAKE_NONCOPYABLE(AXObjectCache); WTF_MAKE_FAST_ALLOCATED;
 public:
-    explicit AXObjectCache(const Document*);
+    explicit AXObjectCache(Document&);
     ~AXObjectCache();
 
     static AXObject* focusedUIElementForPage(const Page*);
 
     // Returns the root object for the entire document.
     AXObject* rootObject();
 
     // For AX objects with elements that back them.
     AXObject* getOrCreate(RenderObject*);
     AXObject* getOrCreate(Widget*);
@@ skipping 111 matching lines @@
     void postPlatformNotification(AXObject*, AXNotification);
     void textChanged(AXObject*);
     void labelChanged(Element*);
 
     // This is a weak reference cache for knowing if Nodes used by TextMarkers are valid.
     void setNodeInUse(Node* n) { m_textMarkerNodes.add(n); }
     void removeNodeForUse(Node* n) { m_textMarkerNodes.remove(n); }
     bool isNodeInUse(Node* n) { return m_textMarkerNodes.contains(n); }
 
 private:
-    Document* m_document;
+    Document& m_document;
     HashMap<AXID, RefPtr<AXObject> > m_objects;
     HashMap<RenderObject*, AXID> m_renderObjectMapping;
     HashMap<Widget*, AXID> m_widgetObjectMapping;
     HashMap<Node*, AXID> m_nodeObjectMapping;
     HashMap<AbstractInlineTextBox*, AXID> m_inlineTextBoxObjectMapping;
     HashSet<Node*> m_textMarkerNodes;
     OwnPtr<AXComputedObjectAttributeCache> m_computedObjectAttributeCache;
     static bool gAccessibilityEnabled;
     static bool gInlineTextBoxAccessibility;
 
     HashSet<AXID> m_idsInUse;
 
     Timer<AXObjectCache> m_notificationPostTimer;
     Vector<pair<RefPtr<AXObject>, AXNotification> > m_notificationsToPost;
     void notificationPostTimerFired(Timer<AXObjectCache>*);
 
     static AXObject* focusedImageMapUIElement(HTMLAreaElement*);
 
     AXID getAXID(AXObject*);
 };
 
 bool nodeHasRole(Node*, const String& role);
 // This will let you know if aria-hidden was explicitly set to false.
 bool isNodeAriaVisible(Node*);
 
 }
 
 #endif