They probable mean safe code like this:

    class FontFeatureValuesMapIterationSource final
    : public PairSyncIterable<CSSFontFeatureValuesMap>::IterationSource {
    public:
    FontFeatureValuesMapIterationSource(const CSSFontFeatureValuesMap& map,
                                      const FontFeatureAliases* aliases)
      : map_(map), aliases_(aliases), iterator_(aliases->begin()) {}