Reduce scope of MediaStreamTrack transfer to DedicatedWorker for now

[jan-ivar]

The MediaStreamTrack WebIDL in § 5.1 Transferable MediaStreamTrack reads:

[Exposed=(Window,Worker), Transferable]

...which is short for:

[Exposed=(Window,DedicatedWorker,SharedWorker)), Transferable]

While use cases exist for DedicatedWorker (mediacapture-transform), there are currently no use cases for SharedWorker.

The latter also lacks implementation: Safari 18 only implements transfer to DedicatedWorker, which is what Firefox plans as well.

Others have expressed difficulty with transfer, so reducing the scope for now seems reasonable. We can always extend it later.

Proposal:

[Exposed=(Window,DedicatedWorker)), Transferable]

[dontcallmedom-bot]

This issue had an associated resolution in WebRTC December 2024 meeting – 10 December 2024 ([mediacapture-extensions] Reduce scope of MediaStreamTrack transfer to DedicatedWorker for now):

RESOLUTION: Proceed with reducing scope of MediaStremaTrack to DedicatedWorker