[mediacapture-main] Pull Request: Add definition to track-ending algorithm

Pehrsons has just submitted a new pull request for https://github.com/w3c/mediacapture-main:

== Add definition to track-ending algorithm ==
The algorithm for ending tracks for reasons other than stop() doesn't
have a definition. Having one lets us reference it from this and other
specifications.

This helps w3c/mediacapture-fromelement#81.

See https://github.com/w3c/mediacapture-main/pull/601