Have Document::topDocument() return a reference

Source/core/frame/LocalFrame.cpp

 /*
  * Copyright (C) 1998, 1999 Torben Weis <weis@kde.org>
  *                     1999 Lars Knoll <knoll@kde.org>
  *                     1999 Antti Koivisto <koivisto@kde.org>
  *                     2000 Simon Hausmann <hausmann@kde.org>
  *                     2000 Stefan Schimanski <1Stein@gmx.de>
  *                     2001 George Staikos <staikos@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2005 Alexey Proskuryakov <ap@nypop.com>
  * Copyright (C) 2008 Nokia Corporation and/or its subsidiary(-ies)
@@ skipping 369 matching lines @@
 {
     // We should never be detatching the page during a Layout.
     RELEASE_ASSERT(!m_view || !m_view->isInPerformLayout());
     m_host = 0;
 }
 
 void LocalFrame::disconnectOwnerElement()
 {
     if (ownerElement()) {
         if (Document* doc = document())
-            doc->topDocument()->clearAXObjectCache();
+            doc->topDocument().clearAXObjectCache();
         ownerElement()->clearContentFrame();
         if (page())
             page()->decrementSubframeCount();
     }
     m_frameInit->setOwnerElement(0);
 }
 
 bool LocalFrame::isMainFrame() const
 {
     Page* page = this->page();
@@ skipping 339 matching lines @@
 {
     if (!m_host)
         return 0;
 
     double ratio = m_host->deviceScaleFactor();
     ratio *= pageZoomFactor();
     return ratio;
 }
 
 } // namespace WebCore