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

Source/core/dom/Element.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Peter Kelly (pmk@post.com)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2013 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
@@ skipping 866 matching lines @@
     ASSERT(hasID());
     return elementData()->idForStyleResolution();
 }
 
 inline bool Element::isIdAttributeName(const QualifiedName& attributeName) const
 {
     // FIXME: This check is probably not correct for the case where the document has an id attribute
     // with a non-null namespace, because it will return false, a false negative, if the prefixes
     // don't match but the local name and namespace both do. However, since this has been like this
     // for a while and the code paths may be hot, we'll have to measure performance if we fix it.
-    return attributeName == document()->idAttributeName();
+    return attributeName == document().idAttributeName();
 }
 
 inline const AtomicString& Element::getIdAttribute() const
 {
-    return hasID() ? fastGetAttribute(document()->idAttributeName()) : nullAtom;
+    return hasID() ? fastGetAttribute(document().idAttributeName()) : nullAtom;
 }
 
 inline const AtomicString& Element::getNameAttribute() const
 {
     return hasName() ? fastGetAttribute(HTMLNames::nameAttr) : nullAtom;
 }
 
 inline const AtomicString& Element::getClassAttribute() const
 {
     if (!hasClass())
         return nullAtom;
     if (isSVGElement())
         return getAttribute(HTMLNames::classAttr);
     return fastGetAttribute(HTMLNames::classAttr);
 }
 
 inline void Element::setIdAttribute(const AtomicString& value)
 {
-    setAttribute(document()->idAttributeName(), value);
+    setAttribute(document().idAttributeName(), value);
 }
 
 inline const SpaceSplitString& Element::classNames() const
 {
     ASSERT(hasClass());
     ASSERT(elementData());
     return elementData()->classNames();
 }
 
 inline size_t Element::attributeCount() const
@@ skipping 164 matching lines @@
 
 inline const Attribute* ElementData::attributeItem(unsigned index) const
 {
     RELEASE_ASSERT(index < length());
     return attributeBase() + index;
 }
 
 } // namespace
 
 #endif