Oilpan: Make the Node hierarchy RefCountedGarbageCollected instead of TreeShared.

Source/core/dom/shadow/ElementShadow.cpp

Index: Source/core/dom/shadow/ElementShadow.cpp
diff --git a/Source/core/dom/shadow/ElementShadow.cpp b/Source/core/dom/shadow/ElementShadow.cpp
index ba11a7035476768c2f723cc0defafba3f50454b5..8be13446f18645649efdd25a428a4ea025891357 100644
--- a/Source/core/dom/shadow/ElementShadow.cpp
+++ b/Source/core/dom/shadow/ElementShadow.cpp
@@ -135,7 +135,9 @@ ElementShadow::ElementShadow()
 
 ElementShadow::~ElementShadow()
 {
+#if !ENABLE(OILPAN)
     removeDetachedShadowRoots();
+#endif
 }
 
 ShadowRoot& ElementShadow::addShadowRoot(Element& shadowHost, ShadowRoot::ShadowRootType type)
@@ -350,4 +352,12 @@ void ElementShadow::clearDistribution()
         root->setShadowInsertionPointOfYoungerShadowRoot(nullptr);
 }
 
+void ElementShadow::trace(Visitor* visitor)
+{
+    // Shadow roots are linked with previous and next pointers which are traced.
+    // It is therefore enough to trace one of the shadow roots here and the
+    // rest will be traced from there.
+    visitor->trace(m_shadowRoots.head());
+}
+
 } // namespace