Oilpan: make DOMWindowProperty a GC mixin.

Source/core/timing/PerformanceTiming.h

Index: Source/core/timing/PerformanceTiming.h
diff --git a/Source/core/timing/PerformanceTiming.h b/Source/core/timing/PerformanceTiming.h
index 19f676fa10b86d0aa8f3a221b213171db7995306..8e61262f6fd0970c83f6becaab700de731293a5a 100644
--- a/Source/core/timing/PerformanceTiming.h
+++ b/Source/core/timing/PerformanceTiming.h
@@ -45,8 +45,9 @@ struct DocumentTiming;
 class LocalFrame;
 class ResourceLoadTiming;
 
-class PerformanceTiming FINAL : public RefCountedWillBeGarbageCollectedFinalized<PerformanceTiming>, public ScriptWrappable, public DOMWindowProperty {
+class PerformanceTiming FINAL : public RefCountedWillBeGarbageCollected<PerformanceTiming>, public ScriptWrappable, public DOMWindowProperty {
     DEFINE_WRAPPERTYPEINFO();
+    WILL_BE_USING_GARBAGE_COLLECTED_MIXIN(PerformanceTiming);
 public:
     static PassRefPtrWillBeRawPtr<PerformanceTiming> create(LocalFrame* frame)
     {
@@ -75,7 +76,7 @@ public:
     unsigned long long loadEventStart() const;
     unsigned long long loadEventEnd() const;
 
-    void trace(Visitor*) { }
+    virtual void trace(Visitor*) OVERRIDE;
 
 private:
     explicit PerformanceTiming(LocalFrame*);