MSE: Add StreamParser buffer remuxing utility and tests

media/formats/mp3/mp3_stream_parser.cc

Index: media/formats/mp3/mp3_stream_parser.cc
diff --git a/media/formats/mp3/mp3_stream_parser.cc b/media/formats/mp3/mp3_stream_parser.cc
index 832b9f92d7c011694d098e7a36bd70ed621bb3c4..2127cc48216531498d8a89f520b41b4e767d64a1 100644
--- a/media/formats/mp3/mp3_stream_parser.cc
+++ b/media/formats/mp3/mp3_stream_parser.cc
@@ -418,8 +418,12 @@ int MP3StreamParser::ParseMP3Frame(const uint8* data,
       return -1;
   }
 
+  // TODO(wolenetz/acolwell): Validate and use a common cross-parser TrackId
+  // type and allow multiple audio tracks, if applicable. See
+  // https://crbug.com/341581.
   scoped_refptr<StreamParserBuffer> buffer =
-      StreamParserBuffer::CopyFrom(data, frame_size, true);
+      StreamParserBuffer::CopyFrom(data, frame_size, true,
+                                   DemuxerStream::AUDIO, 0);
   buffer->set_timestamp(timestamp_helper_->GetTimestamp());
   buffer->set_duration(timestamp_helper_->GetFrameDuration(sample_count));
   buffers->push_back(buffer);