Have Element::ensureUserAgentShadowRoot() return a reference

Source/core/html/HTMLProgressElement.cpp

 /*
  * 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 127 matching lines @@
 {
     m_value->setWidthPercentage(position() * 100);
     if (RenderProgress* render = renderProgress()) {
         bool wasDeterminate = render->isDeterminate();
         render->updateFromElement();
         if (wasDeterminate != isDeterminate())
             didAffectSelector(AffectedSelectorIndeterminate);
     }
 }
 
-void HTMLProgressElement::didAddUserAgentShadowRoot(ShadowRoot* root)
+void HTMLProgressElement::didAddUserAgentShadowRoot(ShadowRoot& root)
 {
     ASSERT(!m_value);
 
     RefPtr<ProgressInnerElement> inner = ProgressInnerElement::create(document());
     inner->setPart(AtomicString("-webkit-progress-inner-element", AtomicString::ConstructFromLiteral));
-    root->appendChild(inner);
+    root.appendChild(inner);
 
     RefPtr<ProgressBarElement> bar = ProgressBarElement::create(document());
     bar->setPart(AtomicString("-webkit-progress-bar", AtomicString::ConstructFromLiteral));
     RefPtr<ProgressValueElement> value = ProgressValueElement::create(document());
     m_value = value.get();
     m_value->setPart(AtomicString("-webkit-progress-value", AtomicString::ConstructFromLiteral));
     m_value->setWidthPercentage(HTMLProgressElement::IndeterminatePosition * 100);
     bar->appendChild(m_value);
 
     inner->appendChild(bar);
 }
 
 bool HTMLProgressElement::shouldAppearIndeterminate() const
 {
     return !isDeterminate();
 }
 
 } // namespace