Fix Frame::isMainFrame() to be compatible with provisional frames

Source/core/frame/Frame.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 98 matching lines @@
     return nullptr;
 }
 
 FrameHost* Frame::host() const
 {
     return m_host;
 }
 
 bool Frame::isMainFrame() const
 {
-    Page* page = this->page();
-    return page && this == page->mainFrame();
+    return !tree().parent();
 }
 
 bool Frame::isLocalRoot() const
 {
     if (isRemoteFrame())
         return false;
 
     if (!tree().parent())
         return true;
 
@@ skipping 165 matching lines @@
 
     if (m_owner) {
         if (m_owner->isLocal())
             toHTMLFrameOwnerElement(m_owner)->setContentFrame(*this);
     } else {
         page()->setMainFrame(this);
     }
 }
 
 } // namespace blink