Need EncodedChunk to have video metadata to support WebRTC

[alvestrand]

The WebRTC Encoded Transform spec's VideoFrame object (https://w3c.github.io/webrtc-encoded-transform/#rtcencodedvideoframe) has a getMetadata function that returns information about the frame that is produced by the encoder, and needs to be passed on to consumers (and vice versa for decoders).

If we're going to harmonize RTCEncodedVideoFrame with EncodedVideoChunk, we need some way to provide this functionality.

(This bug is created as part of bug triage on webrtc-encoded-transform; all bugs that are about buffer formats should move here, I think.)

[aboba]

@alvestrand MetaData Proposal: https://docs.google.com/document/d/1O6kxAQPLWhCRHkH-6zutcqEUOUWRS6NFM2CQAvi1VTg/

Do you need anything beyond that?

[aboba]

@alvestrand Looks like we need to plan for metadata extensibility, so moving the SVC metadata into its own svc dictionary probably makes sense.

[aboba]

@alvestrand @Djuffin @padenot Do we still believe that changes to EncodedChunk are needed to support WebRTC? Or is the required metadata being added to RTCEncoded{Audio, Video}Frame instead?

[padenot]

For WebRTC folks to answer.

[aboba]

@alvestrand Is this still an issue, or can we close this?