UTS: Use synchronizing publicTask for child spawns.
UTS: Use synchronizing publicTask for child spawns.

Fixes GCE issues caused by stolen tasks being completed on a different
GCE than they were spawned.

buaasun/grappa,uwsampa/grappa,alexfrolov/grappa,uwsampa/grappa,buaasun/grappa,buaasun/grappa,uwsampa/grappa,buaasun/grappa,buaasun/grappa,alexfrolov/grappa,uwsampa/grappa,buaasun/grappa,uwsampa/grappa,alexfrolov/grappa,uwsampa/grappa,uwsampa/grappa,alexfrolov/grappa,buaasun/grappa,alexfrolov/grappa,alexfrolov/grappa,alexfrolov/grappa