Ensure FFmpegDemuxer WeakPtrs are created on the right thread.

media/filters/ffmpeg_demuxer.h

 // Copyright (c) 2012 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.
 
 // Implements the Demuxer interface using FFmpeg's libavformat.  At this time
 // will support demuxing any audio/video format thrown at it.  The streams
 // output mime types audio/x-ffmpeg and video/x-ffmpeg and include an integer
 // key FFmpegCodecID which contains the CodecID enumeration value.  The CodecIDs
 // can be used to create and initialize the corresponding FFmpeg decoder.
 //
@@ skipping 336 matching lines @@
   std::unique_ptr<BlockingUrlProtocol> url_protocol_;
   std::unique_ptr<FFmpegGlue> glue_;
 
   const EncryptedMediaInitDataCB encrypted_media_init_data_cb_;
 
   const MediaTracksUpdatedCB media_tracks_updated_cb_;
 
   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> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(FFmpegDemuxer);
 };
 
 }  // namespace media
 
 #endif  // MEDIA_FILTERS_FFMPEG_DEMUXER_H_