FFmpegDemuxer should fall back to disabled streams for seeking

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 6c065de24f66136852e05dc2788173b24d1cf8b1..5d155e6f99952f7eb6e4a428f9ba328da706b353 100644
--- a/media/filters/ffmpeg_demuxer.h
+++ b/media/filters/ffmpeg_demuxer.h
@@ -328,10 +328,15 @@ class MEDIA_EXPORT FFmpegDemuxer : public Demuxer {
   // based timeline.
   base::TimeDelta start_time_;
 
+  // Finds the stream with the lowest known start time (i.e. not kNoTimestamp
+  // start time) with enabled status matching |enabled|.
+  FFmpegDemuxerStream* FindStreamWithLowestStartTimestamp(bool enabled);
+
   // Finds a preferred stream for seeking to |seek_time|. Preference is
   // typically given to video streams, unless the |seek_time| is earlier than
   // the start time of the video stream. In that case a stream with the earliest
-  // start time is preferred. Disabled streams are not considered.
+  // start time is preferred. Disabled streams are considered only as the last
+  // fallback option.
   FFmpegDemuxerStream* FindPreferredStreamForSeeking(base::TimeDelta seek_time);
 
   // The Time associated with timestamp 0. Set to a null