message: Fix for stringpiece build failure
diff: @@ -2054,7 +2054,7 @@ class FactoryKeyCreator {
  const char delimiter = 'x';
  const int kMaxKeyLength = 256;
  void Append(StringPiece s) {
  - key_.append(s.ToString());
  + key_.append(string(s));
  key_.append(1, delimiter);
  }
  };
  