To better align with the Chunking Handler, the Text Segmenter
now outputs the SegmentInfo at the end of the segment rather
than the start.
Change-Id: If69ab951947d00779b4b63a52c4b6662bbdc4c0d
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