Oilpan: add missing leftmost trace()s for GC mixins.

Source/core/html/track/AudioTrack.h

Index: Source/core/html/track/AudioTrack.h
diff --git a/Source/core/html/track/AudioTrack.h b/Source/core/html/track/AudioTrack.h
index f282cd8a42c4d969d193b59acff8134c2ee64139..122d473acdff798799e5ba9841b5dc08fdfb2903 100644
--- a/Source/core/html/track/AudioTrack.h
+++ b/Source/core/html/track/AudioTrack.h
@@ -18,7 +18,9 @@ public:
     {
         return adoptRefWillBeNoop(new AudioTrack(id, kind, label, language, enabled));
     }
+
     virtual ~AudioTrack();
+    virtual void trace(Visitor*) override;
 
     bool enabled() const { return m_enabled; }
     void setEnabled(bool);