If a reviewing user isn't already in the ReviewerSet of a change, and
they post a message without voting, they expect to be notified if anyone
replies to their review. In this circumstance we can simply add the user
to the CC list.
Bug: Issue 4638
Change-Id: Ia5e6b9791e5b341bbc524dd421fa7eb32dbc733d
d4c8e1b827