Have Frame::tree() return a reference

Source/core/page/Chrome.cpp

 /*
  * Copyright (C) 2006, 2007, 2009, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2012, Samsung Electronics. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
@@ skipping 140 matching lines @@
     m_client->show(policy);
 }
 
 bool Chrome::canRunModal() const
 {
     return m_client->canRunModal();
 }
 
 static bool canRunModalIfDuringPageDismissal(Page* page, ChromeClient::DialogType dialog, const String& message)
 {
-    for (Frame* frame = page->mainFrame(); frame; frame = frame->tree()->traverseNext()) {
+    for (Frame* frame = page->mainFrame(); frame; frame = frame->tree().traverseNext()) {
         Document::PageDismissalType dismissal = frame->document()->pageDismissalEventBeingDispatched();
         if (dismissal != Document::NoDismissal)
             return page->chrome().client().shouldRunModalDialogDuringPageDismissal(dialog, message, dismissal);
     }
     return true;
 }
 
 bool Chrome::canRunModalNow() const
 {
     return canRunModal() && canRunModalIfDuringPageDismissal(m_page, ChromeClient::HTMLDialog, String());
@@ skipping 235 matching lines @@
 }
 
 void Chrome::notifyPopupOpeningObservers() const
 {
     const Vector<PopupOpeningObserver*> observers(m_popupOpeningObservers);
     for (size_t i = 0; i < observers.size(); ++i)
         observers[i]->willOpenPopup();
 }
 
 } // namespace WebCore