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

Source/core/html/track/AudioTrack.cpp

 // 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.
 
 #include "config.h"
 #include "core/html/track/AudioTrack.h"
 
 #include "core/html/HTMLMediaElement.h"
 
 namespace blink {
 
 AudioTrack::AudioTrack(const String& id, const AtomicString& kind, const AtomicString& label, const AtomicString& language, bool enabled)
     : TrackBase(TrackBase::AudioTrack, label, language, id)
     , m_enabled(enabled)
 {
     setKind(kind);
 }
 
 AudioTrack::~AudioTrack()
 {
 }
 
+void AudioTrack::trace(Visitor* visitor)
+{
+    TrackBase::trace(visitor);
+}
+
 void AudioTrack::setEnabled(bool enabled)
 {
     if (enabled == m_enabled)
         return;
 
     m_enabled = enabled;
 
     if (mediaElement())
         mediaElement()->audioTrackChanged();
 }
@@ skipping 44 matching lines @@
         || (kind == commentaryKeyword())
         || (kind == emptyAtom);
 }
 
 AtomicString AudioTrack::defaultKind() const
 {
     return emptyAtom;
 }
 
 }