Google's internal version of the gRPC codebase uses a version of
protobuf with a different namespace and include paths. This commit makes
parse_json.cc rely on config_protobuf.h for protobuf types so that it
will work properly in that environment. I also replaced a call to
Status::error_code() with a call to Status::code() instead. This way the
code will work with absl::Status, which has a somewhat different API
compared to google::protobuf::util::Status.
Esun Kim