[MSE][GStreamer] Avoid changing ref count of trackId string from non-main thread

StringImpl's ref count in not thread safe.

Author: emutavchi

Source/WebCore/platform/graphics/gstreamer/mse/MediaSourceTrackGStreamer.h

@@ -45,7 +45,7 @@ class MediaSourceTrackGStreamer final: public ThreadSafeRefCounted<MediaSourceTr
     virtual ~MediaSourceTrackGStreamer();
 
     TrackPrivateBaseGStreamer::TrackType type() const { return m_type; }
-    AtomString trackId() const { return m_trackId; }
+    const AtomString& trackId() const { return m_trackId; }
     GRefPtr<GstCaps>& initialCaps() { return m_initialCaps; }
     DataMutex<TrackQueue>& queueDataMutex() { return m_queueDataMutex; }
 

Source/WebCore/platform/graphics/gstreamer/mse/SourceBufferPrivateGStreamer.cpp

@@ -185,12 +185,12 @@ void SourceBufferPrivateGStreamer::notifyClientWhenReadyForMoreSamples(const Ato
     ASSERT(isMainThread());
     ASSERT(m_tracks.contains(trackId));
     MediaSourceTrackGStreamer* track = m_tracks.get(trackId);
-    track->notifyWhenReadyForMoreSamples([weakPtr = WeakPtr { *this }, this, trackId]() mutable {
-        RunLoop::main().dispatch([weakPtr = WTFMove(weakPtr), this, trackId]() {
+    track->notifyWhenReadyForMoreSamples([weakPtr = WeakPtr { *this }, this, trackId = trackId.string().isolatedCopy()]() mutable {
+        RunLoop::main().dispatch([weakPtr = WTFMove(weakPtr), this, trackId = WTFMove(trackId)]() {
             if (!weakPtr)
                 return;
             if (!m_hasBeenRemovedFromMediaSource)
-                provideMediaData(trackId);
+                provideMediaData(AtomString{ trackId });
         });
     });
 }