This is used to detect what data is still needed to maintain or resume
playback based on the current playback position. This is the last piece
that was preventing YouTube MSE from working.
The segments are parsed for the SourceBufferProcessor by the
WebMByteStreamParser. It parses the initialization segment to update
its internal set of tracks, then SourceBufferProcessor/SourceBuffer set
them up for playback. When a media segment is received, it also parses
as much of it as is available, returning all the coded frames found so
far. SourceBufferProcessor then tells TrackBufferDemuxer to remove any
overlapping frames and insert the new ones.
TrackBufferDemuxer implements the Demuxer interface in terms of the
coded frame store maintained by the SourceBufferProcessor. It returns
the frames in decode order when requested by a data provider. When a
is needed, it finds the keyframe prior to the target timestamp, and
checks that there are no gaps in data up to the target timestamp. If
there are any gaps, it blocks until the gaps are gone.
