Instead of acquiring and releasing a barrier using an up/down
counter, we post a message to the queue that represents the
barrier. This is a more natural representation of the barrier
and better matches what we want to do with it: stall messages
behind the barrier in the queue while allowing messages earlier
in the queue to run as usual.
Refactored the MessageQueue a little bit to simplify the quit
logic and to better encapsulate the invariant that all
messages within the queue must have a valid target. Messages
without targets are used to represent barriers.
Bug: 5721047
Change-Id: Id297d9995474b5e3f17d24e302c58168e0a00394
0f85ce3837