To reproduce this tree, to:

1. git clone https://github.com/nanopb/nanopb.git in a convenient directory
2. git checkout <tag> (current is 0.3.6, commithash eb0e73ca) Note that we want the same nanopb release used by gRPC upstream for the version in third_party/grpc.
3. rm -fr <nanopb git tree>/tests
4. `cp -R /* third_party/nanopb
5. Update BUILD and README.bazel.md if necessary.