WebVTTCaptionFile constructor
      
      WebVTTCaptionFile(
    
- String fileContents
Parses a string into a ClosedCaptionFile, assuming fileContents is in
the WebVTT file format.
Implementation
WebVTTCaptionFile(String fileContents)
    : _captions = _parseCaptionsFromWebVTTString(fileContents);