Ensure FFmpegDemuxer WeakPtrs are created on the right thread.

media/filters/ffmpeg_demuxer.h

Index: media/filters/ffmpeg_demuxer.h
diff --git a/media/filters/ffmpeg_demuxer.h b/media/filters/ffmpeg_demuxer.h
index 7e65cb49cd2c24488ec30d1d6879fe09e0540406..bdb71490e27693c8137c547c273e70123efe9908 100644
--- a/media/filters/ffmpeg_demuxer.h
+++ b/media/filters/ffmpeg_demuxer.h
@@ -354,6 +354,8 @@ class MEDIA_EXPORT FFmpegDemuxer : public Demuxer {
   std::map<MediaTrack::Id, const DemuxerStream*> track_id_to_demux_stream_map_;
 
   // NOTE: Weak pointers must be invalidated before all other member variables.
+  base::WeakPtr<FFmpegDemuxer> weak_this_;
+  base::WeakPtrFactory<FFmpegDemuxer> cancel_pending_seek_factory_;
   base::WeakPtrFactory<FFmpegDemuxer> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(FFmpegDemuxer);