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

Source/core/html/track/WebVTTElement.cpp

 /*
  * Copyright (C) 2013 Apple 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:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 60 matching lines @@
 {
 }
 
 PassRefPtr<WebVTTElement> WebVTTElement::create(WebVTTNodeType nodeType, Document* document)
 {
     return adoptRef(new WebVTTElement(nodeType, document));
 }
 
 PassRefPtr<Element> WebVTTElement::cloneElementWithoutAttributesAndChildren()
 {
-    RefPtr<WebVTTElement> clone = create(static_cast<WebVTTNodeType>(m_webVTTNodeType), document());
+    RefPtr<WebVTTElement> clone = create(static_cast<WebVTTNodeType>(m_webVTTNodeType), &document());
     clone->setLanguage(m_language);
     return clone;
 }
 
 PassRefPtr<HTMLElement> WebVTTElement::createEquivalentHTMLElement(Document* document)
 {
     RefPtr<HTMLElement> htmlElement;
     switch (m_webVTTNodeType) {
     case WebVTTNodeTypeClass:
     case WebVTTNodeTypeLanguage:
@@ skipping 20 matching lines @@
     default:
         ASSERT_NOT_REACHED();
     }
 
     htmlElement.get()->setAttribute(HTMLNames::classAttr, getAttribute(HTMLNames::classAttr));
     return htmlElement;
 }
 
 } // namespace WebCore