API: add VTTCue (#2237)
API: add VTTCue (#2237)

* Web APIs: add VTTCue

Sources:
1. W3C WebVTT spec (skeleton):
   https://w3c.github.io/webvtt/#the-vttcue-interface
2. Manual test in:
   - Chromium 66
   - Firefox 59
   - Opera 50

* Address review comments; sort browsers/features

chrisdavidmills/browser-compat-data,mdn/browser-compat-data,chrisdavidmills/browser-compat-data,mdn/browser-compat-data,teoli2003/browser-compat-data