webrtc: Persist ICE candidates to new local descriptions after renegotiation
The following discussion from !1053 (merged) should be addressed:
The answer to this question is yes. The ice candidates should persist until a new ICE candidate gathering phase is performed (rules for when that happens are in the JSEP spec). Not all SDP renegotiations will perform or require a new ICE candidate gathering phase.
webrtcbin needs to keep the internal list of active ICE candidates and merge them into new SDPs, or perhaps as an alternative implementation - merge them to SDPs as they are retrieved by the current-local-description and current-pending-description properties.