PSA: a=extmap-allow-mixed will be offered by default

[Emil Lundmark]

As of commit 801c9995c86ca7cf3227a27ad9e75f04e1ba235e [1] the SDP
attribute extmap-allow-mixed [2] will be offered by default on session
level.

This attribute allows mixing one- and two-byte RTP header extensions.
Historically, WebRTC has only supported the one-byte header form. The
two-byte header form allows for a wider range of local IDs, thus
increasing the total number of extensions that can be mapped. It also
allows for data lengths up to 255 bytes. For more details, refer to RFC
8285.

The SDP attribute extmap-allow-mixed has been supported since Chrome
M71. However, the SDP negotiation in Chrome versions before M71 will
fail if extmap-allow-mixed is offered. From Chrome M89
extmap-allow-mixed will be offered by default [3].

To get the old behavior back in Chrome, pass
{offerExtmapAllowMixed:false} as an argument to the RTCPeerConnection
constructor. Note that this is a non-standard property.

[1] https://webrtc.googlesource.com/src/+/801c9995c86ca7cf3227a27ad9e75f04e1ba235e
[2] https://tools.ietf.org/html/rfc8285#section-6
[3] https://crrev.com/0e30c7df0477fc05689e2bc0564953f2900d7d12