[webrtc-pc] duration and interToneGap should be stored as internal slots of RTCDTMFSender

soareschen has just created a new issue for https://github.com/w3c/webrtc-pc:

== duration and interToneGap should be stored as internal slots of RTCDTMFSender ==
Since they are not defined as attributes, currently they look like closure variables that are captured when Playout task is executed. 

With current definition, when two calls of `insertDTMF()` are made in succession, the `duration` and `interToneGap` of the first call is used even though the `toneBuffer` is overridden in second call.

Please view or discuss this issue at https://github.com/w3c/webrtc-pc/issues/1516 using your GitHub account