[[IceRole]] initialized too late

[jan-ivar]

#2331 initializes [[IceRole]] in SRD(answer), which is too late, and on the wrong transport (RTCDtlsTransport has no such slot):

This should be moved to where transports are first surfaced in SLD. There the note talking about offers will make sense.¹

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_{1. This is all handled in SLD. I've verified with https://jsfiddle.net/jib1/8axfk9ud/ that in the case of "remote offer" the transports aren't instantiated until SLD(answer) in Chrome and Safari.}

[jan-ivar]

...and on the wrong transport (RTCDtlsTransport has no such slot)

What's going on here is that RTCDtlsTransport has an iceTransport attribute pointing to an RTCIceTransport, so creating the former implicitly necessitates creating the latter at the same time, I suppose.

This would probably be cleaner with creation algorithms for both.