Oilpan: trace RenderVTTCue's cue back reference.

Source/core/rendering/RenderVTTCue.h

 /*
  * Copyright (C) 2012 Victor Carbune (victor@rosedu.org)
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 11 matching lines @@
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 #ifndef RenderVTTCue_h
 #define RenderVTTCue_h
 
 #include "core/rendering/RenderBlockFlow.h"
 #include "core/rendering/RenderInline.h"
 #include "platform/geometry/FloatPoint.h"
+#include "platform/heap/Handle.h"
 
 namespace blink {
 
 class VTTCue;
 class VTTCueBox;
 
 class RenderVTTCue final : public RenderBlockFlow {
 public:
     explicit RenderVTTCue(VTTCueBox*);
 
+    virtual void trace(Visitor*) override;
 private:
     virtual void layout() override;
 
     bool isOutside() const;
     bool isOverlapping() const;
     bool shouldSwitchDirection(InlineFlowBox*, LayoutUnit) const;
 
     void moveBoxesByStep(LayoutUnit);
     bool switchDirection(bool&, LayoutUnit&);
 
     bool findFirstLineBox(InlineFlowBox*&);
     bool initializeLayoutParameters(InlineFlowBox*, LayoutUnit&, LayoutUnit&);
     void placeBoxInDefaultPosition(LayoutUnit, bool&);
     void repositionCueSnapToLinesSet();
     void repositionCueSnapToLinesNotSet();
 
-    VTTCue* m_cue;
+    RawPtrWillBeMember<VTTCue> m_cue;
     FloatPoint m_fallbackPosition;
 };
 
 } // namespace blink
 
 #endif // RenderVTTCue_h