Code review comment for lp:~alan-griffiths/mir/rework-BroadcastingSessionEventSink

+ std::shared_ptr<SessionObserver> const session_observer;
does it really need to be shared? Couldn't the ConfigChanger keep a unique_ptr, and pass the raw pointer to the event register/sink?

Admittedly that probably causes more a lot more code churn.

Rest looks fine to me