The parsing specification requires the UA to drop
cues containing malformed cue-times.
(http://dev.w3.org/html5/webvtt/#dfn-collect-a-webvtt-timestamp)
The OnReportError callback has been modified to notify the parser
that we need need to drop such cues which are deemed 'malformed'
according to the timestamp parsing rules.
- Implemented WebVTTLoadListener to manage the webvtt parser.
- TextTrackCue now handles the conversion of webvtt nodes to
anonymous content which will be displayed on the video div
overlay.
- HTMLTrackElement manages the lifetime of the WebVTTLoadListener.
