message: Fix passing StringPiece to proto string setter in dispatcher_state_test.
diff: @@ -79,7 +79,7 @@ Status CreateNamedJob(int64 job_id, int64 dataset_id, NamedJobKey named_job_key,
  }
  Status CreateTask(int64 task_id, int64 job_id, int64 dataset_id,
  - StringPiece worker_address, DispatcherState* state) {
  + const std::string& worker_address, DispatcherState* state) {
  Update update;
  CreateTaskUpdate* create_task = update.mutable_create_task();
  create_task->set_task_id(task_id);
  