Allow RTCDataChannel transfer to dedicated workers

This CL updates RTCDataChannel to expose it in dedicated workers, and
adds the necessary boilerplate to allow its transfer to a workers.

Exposing RTCDataChannel to dedicated workers is possible since the
workers live in the same Renderer process. It is unclear whether
we can support exposing them in SharedWorkers or ServiceWorkers,
since this would require establishing a new PeerConnection (and a new
underlying "native" WebRTC instance) in a different process.

Relevant portion of the webrtc-extension specification:
https://w3c.github.io/webrtc-extensions/#rtcdatachannel-transferable

Bug: 40787712
Change-Id: I5e14fb807c2d598e1db4e4f105f2913ebf4c934a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5749044
Commit-Queue: Florent Castelli <orphis@chromium.org>
Auto-Submit: Thomas Guilbert <tguilbert@chromium.org>
Reviewed-by: Florent Castelli <orphis@chromium.org>
Reviewed-by: Tony Herre <toprice@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1338345}