Fixed crash with Multisample observer

Source/core/page/Page.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013 Apple Inc. All Rights Reserved.
  * Copyright (C) 2008 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  *
  * 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.
  * This library is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 204 matching lines @@
 }
 
 void Page::setMainFrame(PassRefPtr<LocalFrame> mainFrame)
 {
     ASSERT(!m_mainFrame); // Should only be called during initialization
     m_mainFrame = mainFrame;
 }
 
 void Page::documentDetached(Document* document)
 {
+    m_multisamplingChangedObservers.clear();
     m_pointerLockController->documentDetached(document);
     m_contextMenuController->documentDetached(document);
     if (m_validationMessageClient)
         m_validationMessageClient->documentDetached(*document);
 }
 
 bool Page::openedByDOM() const
 {
     return m_openedByDOM;
 }
@@ skipping 308 matching lines @@
     , spellCheckerClient(0)
     , storageClient(0)
 {
 }
 
 Page::PageClients::~PageClients()
 {
 }
 
 } // namespace WebCore