"enableImplicitRollback" is necessary for perfect negotiation algorithm "offerExtmapAllowMixed" is necessary for backward compatibility with legacy clients. Bug: webrtc:12609 Change-Id: I30a5a01c519ca9080a346e2d36b58f7bab28f15a Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/212741 Reviewed-by: Tommi <tommi@webrtc.org> Commit-Queue: Tommi <tommi@webrtc.org> Cr-Commit-Position: refs/heads/master@{#33639}