WebVTT updates

Hello everyone,

I wanted to send out an update regarding WebVTT and to start the ball
rolling on the next steps.

This week I was at FOMS and was speaking with Crunchyroll and they are
investing heavily in WebVTT, which I thought was very cool and wanted to
share.

I've made a pull request[1] against the WebVTT repo to mark the following
as at risk:
- collision avoidance with snap-to-lines false,
- ::cue-region pseudo-element
- :past and :future pseudo-classes

STYLE blocks are not marked as at risk because I have a prototype[2] that
will be fully implemented by the time the 28-day wait period of updating
the CR has finished.

I have also made a branch[3] with test removals to match the at-risk
features. With the test removals of the at-risk features, there is only one
test that fails (see the rendering 2/27/19 sheet on the WebVTT web platform
tests spreadsheet[4]). This one test is because browsers have not yet
implemented `text-wrap: balance`[5] and the WebVTT defines that cues should
have that property set.

I think we should proceed with marking the above features as at-risk, then
working on a new draft that removes the text to get it moved to PR.
Afterwards, we can create a new Draft or even CR with the at-risk features
put back in and work towards a new version of WebVTT.

Thanks,
Gary.

[1]: https://github.com/w3c/webvtt/pull/449
[2]: https://github.com/videojs/vtt.js/pull/35
[3]:
https://github.com/web-platform-tests/wpt/compare/master...gkatsev:webvtt-at-risk?expand=1
[4]:
https://docs.google.com/spreadsheets/d/e/2PACX-1vSHzJtwi_2ijdV7hRUH0Fe8qZeSwMaCLErK7w7U_IFzhzNmW9YwPRWrER1eHR9-4Eo0kqyXJ4G35b1b/pubhtml
[5]: https://www.w3.org/TR/css-text-4/#valdef-text-wrap-balance