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

Source/core/accessibility/AccessibilityImageMapLink.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 48 matching lines @@
 }
 
 AccessibilityObject* AccessibilityImageMapLink::parentObject() const
 {
     if (m_parent)
         return m_parent;
 
     if (!m_mapElement.get() || !m_mapElement->renderer())
         return 0;
 
-    return m_mapElement->document()->axObjectCache()->getOrCreate(m_mapElement->renderer());
+    return m_mapElement->document().axObjectCache()->getOrCreate(m_mapElement->renderer());
 }
 
 AccessibilityRole AccessibilityImageMapLink::roleValue() const
 {
     if (!m_areaElement)
         return LinkRole;
 
     const AtomicString& ariaRole = getAttribute(roleAttr);
     if (!ariaRole.isEmpty())
         return AccessibilityObject::ariaRoleToWebCoreRole(ariaRole);
@@ skipping 69 matching lines @@
     else
         renderer = m_mapElement->renderer();
 
     if (!renderer)
         return LayoutRect();
 
     return m_areaElement->computeRect(renderer);
 }
 
 } // namespace WebCore