This change introduces handlers to output WebVtt text files. There is
only one output but there is a common base to support others.
WebVttOutputHandler which handles all communication with other handlers
and WebVttSegmentedOutputHandler is responsible for listening for events
and choosing when and where to write cues and headers.
Bug: 36138902
Change-Id: I2b13a94262554398e66fee8cf024aa21041ddbab
Aaron Vaage