https://w3c.github.io/webrtc-pc/#dfn-create-an-rtcrtpsender has a step saying For each stream in streams, add stream.id to [[AssociatedMediaStreamIds]] if it's not already there This applies to addTrack and setStreams and the set of streams in addTransceiver. Tests that default to the stream id as sync group add "-sync" as a postfix BUG=webrtc:14769 Change-Id: I806d2fd87a98d50e54709755541f3f1efff1d8ab Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/288701 Commit-Queue: Philipp Hancke <phancke@microsoft.com> Reviewed-by: Harald Alvestrand <hta@webrtc.org> Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org> Cr-Commit-Position: refs/heads/main@{#38942}