goprotobuf: Minor internal change to track internal message indexes.
goprotobuf: Minor internal change to track internal message indexes.

R=adg
CC=golang-dev
https://codereview.appspot.com/7395047

francoishill/protobuf,daemonl/protobuf,golang/protobuf,qinlodestar/protobuf,jeremyletang/protobuf,ekle/protoc-gen-goweb,jrbarron/protobuf,adnaan/protobuf,ArtemTeleshev/go-protobuf,skyportsystems/go-protobuf,csuideal/protobuf,trdata/protobuf,aoeu/protobuf,skyportsystems/go-protobuf,velocity-toolkit/protobuf-1,golang/protobuf,eduardiazf/protobuf,kung-foo/protobuf,alskjstl/protobuf,linkedin-inc/protobuf,hailocab/protobuf,mwitkow-io/protobuf,tamird/protobuf-2,fuchsia-mirror/third_party-github.com-golang-protobuf,ekle/protoc-gen-goweb_test,tamird/protobuf-2,trdata/protobuf,jmptrader/protobuf-1,fuchsia-mirror/third_party-github.com-golang-protobuf