This change creates the webvtt segmenter which will be used for
segmented text webvtt for HLS. It takes a stream of text samples
and injects segment info messages based on the segment duration.
It is possible for samples to extend between segments, if so the
sample will appear twice (once in each segment).
Change-Id: Iae0134ee61cf269948026086520b6d3f8ce3785b