Have Element::ensureUserAgentShadowRoot() return a reference

Source/core/dom/shadow/ElementShadow.h

 /*
  * Copyright (C) 2012 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.
  *     * Neither the name of Google Inc. nor the names of its
@@ skipping 32 matching lines @@
     WTF_MAKE_NONCOPYABLE(ElementShadow); WTF_MAKE_FAST_ALLOCATED;
 public:
     static PassOwnPtr<ElementShadow> create();
     ~ElementShadow();
 
     Element* host() const;
     ShadowRoot* youngestShadowRoot() const { return m_shadowRoots.head(); }
     ShadowRoot* oldestShadowRoot() const { return m_shadowRoots.tail(); }
     ElementShadow* containingShadow() const;
 
-    ShadowRoot* addShadowRoot(Element& shadowHost, ShadowRoot::ShadowRootType);
+    ShadowRoot& addShadowRoot(Element& shadowHost, ShadowRoot::ShadowRootType);
 
     bool applyAuthorStyles() const { return m_applyAuthorStyles; }
     bool didAffectApplyAuthorStyles();
     bool containsActiveStyles() const;
 
     void attach(const Node::AttachContext&);
     void detach(const Node::AttachContext&);
 
     void removeAllEventListeners();
 
@@ skipping 64 matching lines @@
 inline void ElementShadow::distributeIfNeeded()
 {
     if (m_needsDistributionRecalc)
         distribute();
     m_needsDistributionRecalc = false;
 }
 
 } // namespace
 
 #endif