Use case

This file format is commonly used on the web. #25388 was resolved, but the only supported file type right now is SubRip. More specifically, when the video_player plugin is used to display HLS, we want the ability to read captions from the stream and display them in our app.

Example HLS with captions: https://developer.apple.com/streaming/examples/advanced-stream-fmp4.html

Proposal

We should write a subclass of ClosedCaptionFile in the video_player plugin that can read closed caption files in WebVTT format. We also may need to modify the player to support reading captions on the go, as a stream while the video file is being streamed.