Oilpan: make DOMWindowProperty a GC mixin.

Source/core/editing/DOMSelection.h

Index: Source/core/editing/DOMSelection.h
diff --git a/Source/core/editing/DOMSelection.h b/Source/core/editing/DOMSelection.h
index bf2a65c11b54730fbaa9a5e7450bd5ac63149965..f5f4b5b7b78e1910b3f8688adebd29b5879d549f 100644
--- a/Source/core/editing/DOMSelection.h
+++ b/Source/core/editing/DOMSelection.h
@@ -48,8 +48,9 @@ class Range;
 class TreeScope;
 class VisibleSelection;
 
-class DOMSelection FINAL : public RefCountedWillBeGarbageCollectedFinalized<DOMSelection>, public ScriptWrappable, public DOMWindowProperty {
+class DOMSelection FINAL : public RefCountedWillBeGarbageCollected<DOMSelection>, public ScriptWrappable, public DOMWindowProperty {
     DEFINE_WRAPPERTYPEINFO();
+    WILL_BE_USING_GARBAGE_COLLECTED_MIXIN(DOMSelection);
 public:
     static PassRefPtrWillBeRawPtr<DOMSelection> create(const TreeScope* treeScope)
     {
@@ -96,7 +97,7 @@ public:
     // Microsoft Selection Object API
     void empty();
 
-    void trace(Visitor* visitor) { visitor->trace(m_treeScope); }
+    virtual void trace(Visitor*) OVERRIDE;
 
 private:
     explicit DOMSelection(const TreeScope*);