follow-up from #1146. Very high level but this should cover the relevant code paths.

The jingle offer was taken from a current focus offer with a few bits removed:
* ssrc-info element
* web-socket element inside transport
* rtcp-mux element inside transport
* bridge-session element
* bundle element (removed in focus already)

Ssrcs not grouped together in the jingle element
are not recognized by the bridge. When parsing an
sdp with ssrcs that are not listed consecutively
it is important to still group them together
in the jingle element.