1932526 - Rework shutdown monitoring for WebCodecs Decoders/Encoders to ensure objects properly released

Status: NEW

(Core :: Audio/Video: Web Codecs, defect, P3)

Description

[C.M.Chang[:chunmin]]

We may run into similar issues as bug 1927020 since the mShutdownBlocker in both decoder and encoder hold strong references to themselves:
https://searchfox.org/mozilla-central/rev/81ac0fe96e1bd6020408c2ae7eae6e5b5114d1db/dom/media/webcodecs/DecoderTemplate.cpp#861
https://searchfox.org/mozilla-central/rev/2deb9bcf801f9de83d4f30c890d442072b9b6595/dom/media/webcodecs/EncoderTemplate.cpp#1109

It's better to use similar techniques as ShutdownWatcher / ShutdownConsumer pair introduced in bug 1927020 to replace the UniquePtr<media::ShutdownBlockingTicket> mShutdownBlocker in DecoderTemplate and EncoderTemplate. However, the replacement will require adding a nsIAsyncShutdownBlocker on main thread for decoders, so that RemoteMediaDataDecoder can be safely used under the hood.