Fix flakiness in queued_worker_pool_test.
Fix flakiness in queued_worker_pool_test.

crowell/modpagespeed_tmp,crowell/modpagespeed_tmp,crowell/modpagespeed_tmp,crowell/modpagespeed_tmp,crowell/modpagespeed_tmp,crowell/modpagespeed_tmp,crowell/modpagespeed_tmp