Oilpan: move DOM string collection objects to the Oilpan heap.

Source/core/html/HTMLOutputElement.h

Index: Source/core/html/HTMLOutputElement.h
diff --git a/Source/core/html/HTMLOutputElement.h b/Source/core/html/HTMLOutputElement.h
index ba77d11d08c8d5dd5c59094877d5aacf3a6e1eb0..aca78b9b451fa164fa8a93209dc4bafdd051ccaf 100644
--- a/Source/core/html/HTMLOutputElement.h
+++ b/Source/core/html/HTMLOutputElement.h
@@ -51,6 +51,8 @@ public:
 
     virtual bool canContainRangeEndPoint() const OVERRIDE { return false; }
 
+    virtual void trace(Visitor*) OVERRIDE;
+
 private:
     HTMLOutputElement(Document&, HTMLFormElement*);
 
@@ -64,7 +66,7 @@ private:
 
     bool m_isDefaultValueMode;
     String m_defaultValue;
-    RefPtr<DOMSettableTokenList> m_tokens;
+    RefPtrWillBeMember<DOMSettableTokenList> m_tokens;
 };
 
 } // namespace