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

Source/core/html/track/AudioTrack.cpp

Index: Source/core/html/track/AudioTrack.cpp
diff --git a/Source/core/html/track/AudioTrack.cpp b/Source/core/html/track/AudioTrack.cpp
index 136bab48ccf5414e63eb48bd0cc974c322ccdf0e..0b746da527271bc8b34bc997373ecdc0ea8357a5 100644
--- a/Source/core/html/track/AudioTrack.cpp
+++ b/Source/core/html/track/AudioTrack.cpp
@@ -20,6 +20,11 @@ AudioTrack::~AudioTrack()
 {
 }
 
+void AudioTrack::trace(Visitor* visitor)
+{
+    TrackBase::trace(visitor);
+}
+
 void AudioTrack::setEnabled(bool enabled)
 {
     if (enabled == m_enabled)