as of deeb14aeb5c

#   gen_grpc (optional)
#       If true, generates C++ GRPC.

The instantiation of the template fails when gen_grpc is set to true or 1.
> ERROR at //common-mk/proto_library.gni:34:3: Action has no outputs.
>   action_foreach(action_name) {
>   ^----------------------------

Additionally it needs to acquire path of the grpc_cpp_plugin binary to "grpc_cpp_plugin" variable inside the template.