LibWeb: Replace MarkedVector with Vector in AudioTrackList

There's no need to make the m_audio_tracks entries heap roots, we can
visit them instead.

Userland/Libraries/LibWeb/HTML/AudioTrackList.cpp

@@ -119,4 +119,11 @@ WebIDL::CallbackType* AudioTrackList::onremovetrack()
     return event_handler_attribute(HTML::EventNames::removetrack);
 }
 
+void AudioTrackList::visit_edges(JS::Cell::Visitor& visitor)
+{
+    Base::visit_edges(visitor);
+    for (auto const& track : m_audio_tracks)
+        visitor.visit(track);
+}
+
 }