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

Source/core/dom/shadow/ElementShadow.h

Index: Source/core/dom/shadow/ElementShadow.h
diff --git a/Source/core/dom/shadow/ElementShadow.h b/Source/core/dom/shadow/ElementShadow.h
index bab1c2382d34533346ec0fa07d2412f9ee555e30..6953a0599ba26866b946f7de79ebe40a42025a0e 100644
--- a/Source/core/dom/shadow/ElementShadow.h
+++ b/Source/core/dom/shadow/ElementShadow.h
@@ -35,6 +35,7 @@
 #include "wtf/HashMap.h"
 #include "wtf/Noncopyable.h"
 #include "wtf/PassOwnPtr.h"
+#include "wtf/PassRefPtr.h"
 #include "wtf/Vector.h"
 
 namespace WebCore {
@@ -50,7 +51,7 @@ public:
     ShadowRoot* oldestShadowRoot() const { return m_shadowRoots.tail(); }
     ElementShadow* containingShadow() const;
 
-    ShadowRoot* addShadowRoot(Element& shadowHost, ShadowRoot::ShadowRootType);
+    PassRefPtr<ShadowRoot> addShadowRoot(Element& shadowHost, ShadowRoot::ShadowRootType);
 
     bool applyAuthorStyles() const { return m_applyAuthorStyles; }
     bool didAffectApplyAuthorStyles();