Oilpan: Ensure that classes with virtual trace methods always have vtables for their left-most base…

Source/core/html/track/vtt/VTTRegion.h

 /*
  * Copyright (C) 2013 Google Inc.  All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 79 matching lines @@
     void setRegionSettings(const String&);
 
     bool isScrollingRegion() { return m_scroll; }
 
     PassRefPtrWillBeRawPtr<HTMLDivElement> getDisplayTree(Document&);
 
     void appendVTTCueBox(PassRefPtrWillBeRawPtr<VTTCueBox>);
     void displayLastVTTCueBox();
     void willRemoveVTTCueBox(VTTCueBox*);
 
-    void trace(Visitor*);
+    virtual void trace(Visitor*);
 
 private:
     VTTRegion();
 
     void prepareRegionDisplayTree();
 
     // The timer is needed to continue processing when cue scrolling ended.
     void startTimer();
     void stopTimer();
     void scrollTimerFired(Timer<VTTRegion>*);
@@ skipping 39 matching lines @@
     // The timer is used to display the next cue line after the current one has
     // been displayed. It's main use is for scrolling regions and it triggers as
     // soon as the animation for rolling out one line has finished, but
     // currently it is used also for non-scrolling regions to use a single
     // code path.
     Timer<VTTRegion> m_scrollTimer;
 };
 
 } // namespace blink
 #endif