Make Document::implementation() method return reference

Source/core/dom/Document.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  *           (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
@@ skipping 277 matching lines @@
 
     void setReferrerPolicy(ReferrerPolicy);
     ReferrerPolicy referrerPolicy() const { return m_referrerPolicy; }
 
     String outgoingReferrer();
     String outgoingOrigin() const;
 
     void setDoctype(PassRefPtr<DocumentType>);
     DocumentType* doctype() const { return m_docType.get(); }
 
-    DOMImplementation* implementation();
+    DOMImplementation& implementation();
 
     Element* documentElement() const
     {
         return m_documentElement.get();
     }
 
     bool hasManifest() const;
 
     Location* location() const;
 
@@ skipping 1087 matching lines @@
 inline bool Node::isDocumentNode() const
 {
     return this == document();
 }
 
 Node* eventTargetNodeForDocument(Document*);
 
 } // namespace WebCore
 
 #endif // Document_h