Fixed flaky MasterTest.SlavesEndpointTwoSlaves caused by incorrect FUTURE_PROTOBUF usage.
Fixed flaky MasterTest.SlavesEndpointTwoSlaves caused by incorrect
FUTURE_PROTOBUF usage.

Review: https://reviews.apache.org/r/31114

neilconway/mesos,chhsia0/mesos,Aman-Jain-14/customizedMesos,zmalik/mesos,craimbert/mesos,gsantovena/mesos,chhsia0/mesos,andschwa/mesos,craimbert/mesos,kaysoky/mesos,gsantovena/mesos,abudnik/mesos,asamerh4/mesos,andschwa/mesos,jpeach/mesos,kaysoky/mesos,jpeach/mesos,kaysoky/mesos,dforsyth/mesos,gsantovena/mesos,Gilbert88/mesos,andschwa/mesos,reneploetz/mesos,Gilbert88/mesos,abudnik/mesos,gsantovena/mesos,abudnik/mesos,dforsyth/mesos,neilconway/mesos,gsantovena/mesos,chhsia0/mesos,jpeach/mesos,yuquanshan/customizedMesos,reneploetz/mesos,Gilbert88/mesos,neilconway/mesos,zmalik/mesos,Gilbert88/mesos,abudnik/mesos,verizonlabs/mesos,Aman-Jain-14/customizedMesos,yuquanshan/customizedMesos,jpeach/mesos,yuquanshan/customizedMesos,Aman-Jain-14/customizedMesos,craimbert/mesos,Aman-Jain-14/customizedMesos,yuquanshan/customizedMesos,asamerh4/mesos,neilconway/mesos,kaysoky/mesos,gsantovena/mesos,reneploetz/mesos,verizonlabs/mesos,verizonlabs/mesos,asamerh4/mesos,abudnik/mesos,Aman-Jain-14/customizedMesos,verizonlabs/mesos,gsantovena/mesos,dforsyth/mesos,reneploetz/mesos,yuquanshan/customizedMesos,neilconway/mesos,chhsia0/mesos,asamerh4/mesos,jpeach/mesos,jpeach/mesos,andschwa/mesos,dforsyth/mesos,andschwa/mesos,reneploetz/mesos,verizonlabs/mesos,craimbert/mesos,yuquanshan/customizedMesos,craimbert/mesos,kaysoky/mesos,jpeach/mesos,neilconway/mesos,Gilbert88/mesos,zmalik/mesos,chhsia0/mesos,neilconway/mesos,zmalik/mesos,Gilbert88/mesos,craimbert/mesos,Aman-Jain-14/customizedMesos,zmalik/mesos,dforsyth/mesos,asamerh4/mesos,Aman-Jain-14/customizedMesos,andschwa/mesos,andschwa/mesos,kaysoky/mesos,asamerh4/mesos,chhsia0/mesos,verizonlabs/mesos,abudnik/mesos,verizonlabs/mesos,asamerh4/mesos,yuquanshan/customizedMesos,abudnik/mesos,Gilbert88/mesos,dforsyth/mesos,reneploetz/mesos,chhsia0/mesos,zmalik/mesos,zmalik/mesos,kaysoky/mesos,reneploetz/mesos,dforsyth/mesos,craimbert/mesos