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

Source/core/svg/SVGLengthContext.cpp

 /*
  * Copyright (C) 2004, 2005, 2006 Nikolas Zimmermann <zimmermann@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2007 Rob Buis <buis@kde.org>
  * Copyright (C) 2007 Apple Inc. All rights reserved.
  * Copyright (C) Research In Motion Limited 2011. 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 276 matching lines @@
 
     // If an overriden viewport is given, it has precedence.
     if (!m_overridenViewport.isEmpty()) {
         width = m_overridenViewport.width();
         height = m_overridenViewport.height();
         return true;
     }
 
     // SVGLengthContext should NEVER be used to resolve width/height values for <svg> elements,
     // as they require special treatment, due the relationship with the CSS width/height properties.
-    ASSERT(m_context->document()->documentElement() != m_context);
+    ASSERT(m_context->document().documentElement() != m_context);
 
     // Take size from nearest viewport element.
     SVGElement* viewportElement = m_context->viewportElement();
     if (!viewportElement || !viewportElement->isSVGSVGElement())
         return false;
 
     const SVGSVGElement* svg = static_cast<const SVGSVGElement*>(viewportElement);
     FloatSize viewportSize = svg->currentViewBoxRect().size();
     if (viewportSize.isEmpty())
         viewportSize = svg->currentViewportSize();
 
     width = viewportSize.width();
     height = viewportSize.height();
     return true;
 }
 
 }