Adds support for the "stop" action to the MediaSession interface. An action is an event tied specifically to a common media function such as pause or play. The "stop" action handler is called when the site should stop the playback and clear the state if appropriate.
We should add support for "stop" since this action is supported by the platform and allows sites to provide a better experience.
Docs: https://wicg.github.io/mediasession/#dom-mediasessionaction-stop
Samples: https://googlechrome.github.io/samples/media-session/video.html https://googlechrome.github.io/samples/media-session/audio.html
Explainers: https://github.com/WICG/mediasession/blob/master/explainer.md