Oilpan: Make the Node hierarchy RefCountedGarbageCollected instead of TreeShared.

Source/core/dom/UserActionElementSet.cpp

Index: Source/core/dom/UserActionElementSet.cpp
diff --git a/Source/core/dom/UserActionElementSet.cpp b/Source/core/dom/UserActionElementSet.cpp
index 264061fd539b7220366ad1974aba8c9571c93064..0970d27f376d0c0583f6a63aae73c2f6e6e1b93e 100644
--- a/Source/core/dom/UserActionElementSet.cpp
+++ b/Source/core/dom/UserActionElementSet.cpp
@@ -116,4 +116,9 @@ inline void UserActionElementSet::setFlags(Element* element, unsigned flags)
     m_elements.add(element, flags);
 }
 
+void UserActionElementSet::trace(Visitor* visitor)
+{
+    visitor->trace(m_elements);
+}
+
 }