WebM id mapping

Hi,

I was looking at the Sourcing In-band Media Resource Tracks from Media
Containers into
HTML<http://rawgit.com/silviapfeiffer/HTMLSourcingInbandTracks/master/index.html>
spec
and wanted to suggest that the WebM mapping for id be changed to use
TrackNumber instead of TrackUID. My reasoning is that TrackNumber is akin
to the track_ID in ISOBMFF, pid in MPEG2-TS, and serialnum in Ogg. Like all
these other formats it is the actual ID that is used to demux
blocks/samples/packets in the container.  A container without a TrackUID,
while not totally compliant, is still usable because the TrackNumber is
what actually binds the track info to a sequence of blocks. A missing
TrackNumber leads to an unusable file. I think using TrackNumber is more
consistent with your choices for the other formats.

Thanks,
Aaron