PSA: Changes to RTCDTMFSender

[Harald Alvestrand]

If you don’t use DTMF in WebRTC, you can stop reading now.

Chrome’s previous implementation of DTMF support in WebRTC involved using the createDTMFSender() call on an RTCPeerConnection. This approach has been replaced in the spec with a “dtmf” attribute on an RTCRtpSender.

As of Chrome 66, we implement the “dtmf” attribute on the RTCRtpSender according to spec, and this is now the recommended way of getting an RTCDTMFSender. The createDTMFSender() approach still works, but will be deprecated and removed at some point, along with the nonstandard attributes “track”, “duration” and “interToneGap”.

Note: the “track” attribute is not set for DTMFSenders created by accessing the “dtmf” attribute of an RTCRtpSender. This is intentional, as part of the process for deprecating the attribute.

This change is part of the work of bringing our DTMF sender implementation into compliance with the spec.