Milo: master grpc endpoint fix
Milo: master grpc endpoint fix

the writer needs to be closed (flushed) otherwise it can't be read out.

BUG=642461

Review-Url: https://codereview.chromium.org/2349833003

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go