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

Source/core/html/track/AudioTrack.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 AudioTrack_h
 #define AudioTrack_h
 
 #include "bindings/core/v8/ScriptWrappable.h"
 #include "core/html/track/TrackBase.h"
 
 namespace blink {
 
 class AudioTrack final : public NoBaseWillBeGarbageCollectedFinalized<AudioTrack>, public TrackBase, public ScriptWrappable {
     DEFINE_WRAPPERTYPEINFO();
     WILL_BE_USING_GARBAGE_COLLECTED_MIXIN(AudioTrack);
 public:
     static PassRefPtrWillBeRawPtr<AudioTrack> create(const String& id, const AtomicString& kind, const AtomicString& label, const AtomicString& language, bool enabled)
     {
         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);
 
     // Valid kind keywords.
     static const AtomicString& alternativeKeyword();
     static const AtomicString& descriptionsKeyword();
     static const AtomicString& mainKeyword();
     static const AtomicString& mainDescriptionsKeyword();
     static const AtomicString& translationKeyword();
     static const AtomicString& commentaryKeyword();
 
     static bool isValidKindKeyword(const String&);
 
 private:
     AudioTrack(const String& id, const AtomicString& kind, const AtomicString& label, const AtomicString& language, bool enabled);
 
     // TrackBase
     virtual bool isValidKind(const AtomicString& kind) const override { return isValidKindKeyword(kind); }
     virtual AtomicString defaultKind() const override;
 
     bool m_enabled;
 };
 
 DEFINE_TRACK_TYPE_CASTS(AudioTrack, TrackBase::AudioTrack);
 
 } // namespace blink
 
 #endif // AudioTrack_h