Have Document::topDocument() return a reference

Source/core/accessibility/AXRenderObject.cpp

 /*
 * Copyright (C) 2008 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
@@ skipping 213 matching lines @@
     if (!topDoc)
         return 0;
 
     return topDoc->renderView();
 }
 
 Document* AXRenderObject::topDocument() const
 {
     if (!document())
         return 0;
-    return document()->topDocument();
+    return &document()->topDocument();
 }
 
 bool AXRenderObject::shouldNotifyActiveDescendant() const
 {
     // We want to notify that the combo box has changed its active descendant,
     // but we do not want to change the focus, because focus should remain with the combo box.
     if (isComboBox())
         return true;
 
     return shouldFocusActiveDescendant();
@@ skipping 2078 matching lines @@
         if (label && label->renderer()) {
             LayoutRect labelRect = axObjectCache()->getOrCreate(label)->elementRect();
             result.unite(labelRect);
         }
     }
 
     return result;
 }
 
 } // namespace WebCore