Dealing with isolation state mismatches

This is probably best handled in a room, but here goes.

A has isolated streams because it thinks it's making a "private" call.
 (Scare quotes intentional.)

B has regular streams.

A and B try to establish a call.  Nothing in the signaling they are
using (SDP, woo!) indicates that they are screwed.  The browser runs
the O/A exchange and it seems OK, until the DTLS session blows up.

Do we want a signal in SDP for this state?  I think that it would be
nice.  We can put a wee attributey thing on the a=identity line.

Sorry, scratch that, we can request that the RTCWEB working group
consider this as a new requirement on their signaling work.