Utilize PassRefPtr and C++ refs in more places in ShadowRoot code

Source/core/dom/Element.h

Index: Source/core/dom/Element.h
diff --git a/Source/core/dom/Element.h b/Source/core/dom/Element.h
index 1372c4caff53983474ec60c1a93433e54d3976b1..3bd92f6354556f0aaa4b1356530f952c08dbf67f 100644
--- a/Source/core/dom/Element.h
+++ b/Source/core/dom/Element.h
@@ -312,7 +312,7 @@ public:
     bool hasAuthorShadowRoot() const { return shadowRoot(); }
     virtual void didAddShadowRoot(ShadowRoot&);
     ShadowRoot* userAgentShadowRoot() const;
-    ShadowRoot* ensureUserAgentShadowRoot();
+    ShadowRoot& ensureUserAgentShadowRoot();
     const AtomicString& shadowPseudoId() const;
 
     RenderStyle* computedStyle(PseudoId = NOPSEUDO);
@@ -575,7 +575,7 @@ private:
 
     // FIXME: Everyone should allow author shadows.
     virtual bool areAuthorShadowsAllowed() const { return true; }
-    virtual void didAddUserAgentShadowRoot(ShadowRoot*) { }
+    virtual void didAddUserAgentShadowRoot(ShadowRoot&) { }
     virtual bool alwaysCreateUserAgentShadowRoot() const { return false; }
 
     // FIXME: Remove the need for Attr to call willModifyAttribute/didModifyAttribute.