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

Source/core/dom/FullscreenElementStack.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  *           (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
@@ skipping 100 matching lines @@
     RenderFullScreen* m_fullScreenRenderer;
     Timer<FullscreenElementStack> m_fullScreenChangeDelayTimer;
     Deque<RefPtr<Node> > m_fullScreenChangeEventTargetQueue;
     Deque<RefPtr<Node> > m_fullScreenErrorEventTargetQueue;
     LayoutRect m_savedPlaceholderFrameRect;
     RefPtr<RenderStyle> m_savedPlaceholderRenderStyle;
 };
 
 inline bool FullscreenElementStack::isActiveFullScreenElement(const Element* element)
 {
-    FullscreenElementStack* controller = fromIfExists(element->document());
+    FullscreenElementStack* controller = fromIfExists(&element->document());
     if (!controller)
         return false;
     return controller->webkitIsFullScreen() && controller->webkitCurrentFullScreenElement() == element;
 }
 
 inline FullscreenElementStack* FullscreenElementStack::fromIfExists(Document* document)
 {
     if (!document->hasFullscreenElementStack())
         return 0;
     return fromIfExistsSlow(document);
 }
 
 } // namespace WebCore
 
 #endif // FullscreenElementStack_h