message: human_readable_json.h uses the correct switched namespace protobuf
  but the implementation directly references google::protobuf. Fix it
  so that it agrees with its header file.
diff: @@ -20,7 +20,7 @@ limitations under the License.
  namespace tensorflow {
  -Status ProtoToHumanReadableJson(const ::google::protobuf::Message& proto,
  +Status ProtoToHumanReadableJson(const protobuf::Message& proto,
  string* result) {
  #ifdef TENSORFLOW_LITE_PROTOS
  *result = "[human readable output not available on Android]";
  @@ -28,7 +28,7 @@ Status ProtoToHumanReadableJson(const ::google::protobuf::Message& proto,
  #else
  result->clear();
  - auto status = google::protobuf::util::MessageToJsonString(proto, result);
  + auto status = protobuf::util::MessageToJsonString(proto, result);
  if (!status.ok()) {
  // Convert error_msg google::protobuf::StringPiece to
  // tensorflow::StringPiece.
  @@ -41,8 +41,7 @@ Status ProtoToHumanReadableJson(const ::google::protobuf::Message& proto,
  #endif
  }
  -Status HumanReadableJsonToProto(const string& str,
  - ::google::protobuf::Message* proto) {
  +Status HumanReadableJsonToProto(const string& str, protobuf::Message* proto) {
  #ifdef TENSORFLOW_LITE_PROTOS
  return errors::Internal("Cannot parse JSON protos on Android");
  #else
  