[MSE] Rethinking MediaSource.setTrackInfo() and MediaSource.getSourceBuffer()

Hi,

After spending some time looking at the MSE spec, I'm starting to wonder if
we should rethink our decision to create MediaSource.setTrackInfo() and
MediaSource.getSourceBuffer(). If I remember correctly, the primary
motivation for adding these methods was to provide a way to update the
language and kind with data from something like a DASH manifest. I started
to think about what the JavaScript would have to look like to accomplish
this and it doesn't feel very natural. See below:

function setAudioTrackInfo(mediaElement, mediaSource, sourceBuffer,
language, kind) {
  for (var i = 0; i <  mediaElement.audioTracks.length; ++i) {
     var audioTrack = mediaElement.audioTracks[i];
     if (mediaSource.getSourceBuffer(audioTrack) == sourceBuffer) {
       mediaSource.setTrackInfo(audioTrack, language, kind);
     }
  }
}

Something like this feels like it would be much better

function setAudioTrackInfo(sourceBuffer, language, kind) {
  for (var i = 0; i <  sourceBuffer.audioTracks.length; ++i) {
     var audioTrack = sourceBuffer.audioTracks[i];
     audioTrack.language = language;
     audioTrack.kind = kind;
  }
}

I know that the other motivation for adding these methods were to avoid
having to depend on new features in the HTML spec. I understand and agree
with that goal, but I'm starting to feel that the W3C process is shaping
the design here more than the actual use case.

I'd like to propose that we add the following extensions to AudioTrack,
VideoTrack, TextTrack, and SourceBuffer in the MSE spec.

partial interface AudioTrack {
  attribute DOMString kind;
  attribute DOMString language;
  readonly attribute SourceBuffer sourceBuffer;
}

partial interface VideoTrack {
  attribute DOMString kind;
  attribute DOMString language;
  readonly attribute SourceBuffer sourceBuffer;
}

partial interface TextTrack {
  attribute DOMString kind;
  attribute DOMString language;
  readonly attribute SourceBuffer sourceBuffer;
}

partial interface SourceBuffer {
  readonly attribute AudioTrackList audioTracks;
  readonly attribute VideoTrackList videoTracks;
  readonly attribute TextTrackList textTracks;
}

I think this will make accessing/modifying this information more natural
and maintains the goal of not depending on a specific version of the HTML
spec. This simply specifies the requirements for implementations that use
MSE. If HTML.next decides to include the new declarations for xxx.kind and
xxx.language then that is fine as long as the behavior is consistent
between the 2 specs. This shouldn't be a huge problem since only setter
behavior is being defined and editors of the 2 specs can coordinate on the
definition.

Obviously I don't want this to derail the FPWD. That ship has sailed, but
it may be a good idea to file a bug and update the FPWD candidate with a
link to this proposal.

What do people think about this?

Aaron