Now text cues are composed of nested fragments that can be individually
styled. This allows portions of the cue to be bold, etc. The
WebVTT parser doesn't parse the inputs, but the original tags are
preserved in WebVTT output. The WebVTT output will add tags if the
style elements are present in the cue object.
Change-Id: I6abba4175e376e4f753193f7d8cac63e958d3c89