Checking state before proceeding

I need to check that this all compiles properly (my build config isn't suited to lightweight changes like this).  But I think that this is all the checking that we need here.
I don't think these should be MOZ_ASSERTs. The state should be enforced here, rather
than in PCMedia, so these need to be error asserts.

Also, this one is wrong, since you can do SLD in other states see the state machine diagram
http://rtcweb-wg.github.io/jsep/#rfc.section.3.2

You can, for instance, do SLD in have-local-offer.