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

Source/core/html/track/VideoTrack.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef VideoTrack_h
 #define VideoTrack_h
 
 #include "bindings/core/v8/ScriptWrappable.h"
 #include "core/html/track/TrackBase.h"
 
 namespace blink {
 
 class VideoTrack final : public NoBaseWillBeGarbageCollectedFinalized<VideoTrack>, public TrackBase, public ScriptWrappable {
     DEFINE_WRAPPERTYPEINFO();
     WILL_BE_USING_GARBAGE_COLLECTED_MIXIN(VideoTrack);
 public:
     static PassRefPtrWillBeRawPtr<VideoTrack> create(const String& id, const AtomicString& kind, const AtomicString& label, const AtomicString& language, bool selected)
     {
         return adoptRefWillBeNoop(new VideoTrack(id, kind, label, language, selected));
     }
+
     virtual ~VideoTrack();
+    virtual void trace(Visitor*) override;
 
     bool selected() const { return m_selected; }
     void setSelected(bool);
 
     // Set selected to false without notifying the owner media element. Used when
     // another video track is selected, implicitly deselecting this one.
     void clearSelected() { m_selected = false; }
 
     // Valid kind keywords.
     static const AtomicString& alternativeKeyword();
@@ skipping 13 matching lines @@
     virtual AtomicString defaultKind() const override;
 
     bool m_selected;
 };
 
 DEFINE_TRACK_TYPE_CASTS(VideoTrack, TrackBase::VideoTrack);
 
 } // namespace blink
 
 #endif // VideoTrack_h