warning: comparison of integers of different signs: 'int' and 'size_t'

[bpasero]

Building against Electron 1.3.7 I am getting the following warning on macOS:

../src/onig-string.cc:25:40: warning: comparison of integers of different signs: 'int' and 'size_t' (aka 'unsigned long') [-Wsign-compare]
  hasMultiByteChars = (value->Length() != utf8_length_);