Modified the mkvmuxer to write the ChapterStringUID
sub-element of the Chapter Atom element.
Modified the mkvparser to read the ChapterStringUID
sub-element of the chapter atom.
Modified the vttdemux app to write the Cue Identifier
line of the WebVTT cue.
Change-Id: I06fe386f44897ada3fe10cbf89096df104dcf779
vttdemux is a tool for demuxing a webm file containing
WebVTT metadata tracks, extracting the embedded metadata
from each track and storing it as a standalone WebVTT file.
Change-Id: I8897b3dc502c49c92f5b79925939baa5a9490aaa