Remove useless null checks from Node::document() call sites

Source/core/css/resolver/ElementResolveContext.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. 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.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 29 matching lines @@
         , m_parentNode(0)
         , m_rootElementStyle(0)
         , m_elementLinkState(NotInsideLink)
         , m_distributedToInsertionPoint(false)
         , m_resetStyleInheritance(false)
     {
     }
 
     explicit ElementResolveContext(Element*);
 
+    // FIXME: This should return a reference.
     Document* document() const { return m_element->document(); }
     bool isDocumentElement() const { return m_element && m_element == m_element->document()->documentElement(); }
 
     Element* element() const { return m_element; }
     const ContainerNode* parentNode() const { return m_parentNode; }
     RenderStyle* rootElementStyle() const { return m_rootElementStyle; }
     EInsideLink elementLinkState() const { return m_elementLinkState; }
     bool distributedToInsertionPoint() const { return m_distributedToInsertionPoint; }
     bool resetStyleInheritance() const { return m_resetStyleInheritance; }
 
 private:
     Element* m_element;
     ContainerNode* m_parentNode;
     RenderStyle* m_rootElementStyle;
     EInsideLink m_elementLinkState;
     bool m_distributedToInsertionPoint;
     bool m_resetStyleInheritance;
 };
 
 } // namespace WebCore
 
 #endif // ElementResolveContext_h