Officially support Chromium's new documentPictureInPicture API

[jpodwys]

Twilio employee Donal Toomey requested that I create this issue.

I'd like to request official documentPictureInPicture support. Today, Twilio's very nice efficiency features (pausing tracks when a tab is backgrounded) result in pausing all video feeds in a documentPictureInPicture context. I've forked Twilio's SDK to support this. Would love to see official support so we can get back onto the main repo.

I'm also happy to share the changes I made, although they were more brute force than surgical.

Thank you!