Section 4.1.4.2 of JSEP draft is explaining the "rollback" is a special session description type used to rollback the sessions.

But RTCSdpType of WebRTC draft is missing this SDP type.
It is better to add this type to RTCSdpType and explain the same in setLocalDescription/setRemodeDescription, like

. If setLocalDescription/setRemoteDescription is received with RTCSdpType "rollback".
. If the state is stable, return with invalid state exception and abort the following steps.
. Consider the sdp string to be empty string (clamping the string to be empty to avoid exception cases) and rollback the state from current unstable  state to previous stable state.