Added a missing protobuf field to the unversioned definitions.
Added a missing protobuf field to the unversioned definitions.

The new `OfferOperationUpdate` event type was added to the unversioned
scheduler API protobuf definitions, but the corresponding field was not
added to the `Event` message.

Review: https://reviews.apache.org/r/63767/

asamerh4/mesos,dforsyth/mesos,dforsyth/mesos,Gilbert88/mesos,jpeach/mesos,gsantovena/mesos,jpeach/mesos,Gilbert88/mesos,reneploetz/mesos,gsantovena/mesos,gsantovena/mesos,kaysoky/mesos,asamerh4/mesos,chhsia0/mesos,kaysoky/mesos,jpeach/mesos,kaysoky/mesos,kaysoky/mesos,Gilbert88/mesos,abudnik/mesos,abudnik/mesos,dforsyth/mesos,andschwa/mesos,andschwa/mesos,reneploetz/mesos,asamerh4/mesos,jpeach/mesos,asamerh4/mesos,reneploetz/mesos,chhsia0/mesos,chhsia0/mesos,dforsyth/mesos,abudnik/mesos,dforsyth/mesos,gsantovena/mesos,andschwa/mesos,chhsia0/mesos,dforsyth/mesos,abudnik/mesos,Gilbert88/mesos,jpeach/mesos,Gilbert88/mesos,dforsyth/mesos,gsantovena/mesos,andschwa/mesos,chhsia0/mesos,kaysoky/mesos,reneploetz/mesos,kaysoky/mesos,asamerh4/mesos,chhsia0/mesos,kaysoky/mesos,abudnik/mesos,chhsia0/mesos,andschwa/mesos,Gilbert88/mesos,Gilbert88/mesos,asamerh4/mesos,gsantovena/mesos,reneploetz/mesos,reneploetz/mesos,abudnik/mesos,asamerh4/mesos,jpeach/mesos,gsantovena/mesos,andschwa/mesos,andschwa/mesos,jpeach/mesos,abudnik/mesos,reneploetz/mesos