Have Element::ensureUserAgentShadowRoot() return a reference

Source/core/html/HTMLProgressElement.h

 /*
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies).
  *
  * 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,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 44 matching lines @@
     virtual bool supportLabels() const OVERRIDE { return true; }
 
     virtual RenderObject* createRenderer(RenderStyle*);
     RenderProgress* renderProgress() const;
 
     virtual void parseAttribute(const QualifiedName&, const AtomicString&) OVERRIDE;
 
     virtual void attach(const AttachContext& = AttachContext()) OVERRIDE;
 
     void didElementStateChange();
-    virtual void didAddUserAgentShadowRoot(ShadowRoot*) OVERRIDE;
+    virtual void didAddUserAgentShadowRoot(ShadowRoot&) OVERRIDE;
     bool isDeterminate() const;
 
     ProgressValueElement* m_value;
 };
 
 inline bool isHTMLProgressElement(Node* node)
 {
     ASSERT(node);
     return node->hasTagName(HTMLNames::progressTag);
 }
 
 DEFINE_NODE_TYPE_CASTS(HTMLProgressElement, hasTagName(HTMLNames::progressTag));
 
 } // namespace
 
 #endif