Fix V8 function in Value.h

[Zosoled]

https://v8.github.io/api/head/classv8_1_1String.html#a1520592ac160f6c5e79ace641e09c124

[jcoffland]

This is version dependent. Accepting this PR would break builds for other systems. It would have to be wrapped in an ifdef on the v8 version. At what version was this change made? E.g.: https://github.com/CauldronDevelopmentLLC/cbang/blob/master/src/cbang/js/v8/Context.cpp#L54