When running views_mus_unittests, it is possible for the Mus Window Server to send messages during the test TearDown.

WindowTreeClientImpl processes these in a partially destroyed state, which has uncovered some bugs.

However there is no guarantee to receive these, so there may be other issues not yet uncovered by the tests.

The teardown of WindowTreeClientImpl should block on clearing the queue of inflight changes, so that tests don't flake.