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

Source/core/html/LinkResource.cpp

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 34 matching lines @@
 
 LinkResource::~LinkResource()
 {
 }
 
 LinkRequestBuilder::LinkRequestBuilder(HTMLLinkElement* owner)
     : m_owner(owner)
     , m_url(m_owner->getNonEmptyURLAttribute(hrefAttr))
 {
     m_charset = m_owner->getAttribute(charsetAttr);
-    if (m_charset.isEmpty() && m_owner->document()->frame())
-        m_charset = m_owner->document()->charset();
+    if (m_charset.isEmpty() && m_owner->document().frame())
+        m_charset = m_owner->document().charset();
 }
 
 FetchRequest LinkRequestBuilder::build(bool blocking) const
 {
     ResourceLoadPriority priority = blocking ? ResourceLoadPriorityUnresolved : ResourceLoadPriorityVeryLow;
-    return FetchRequest(ResourceRequest(m_owner->document()->completeURL(m_url)), m_owner->localName(), m_charset, priority);
+    return FetchRequest(ResourceRequest(m_owner->document().completeURL(m_url)), m_owner->localName(), m_charset, priority);
 }
 
 } // namespace WebCore