Pass Isolate pointer to v8::String::Utf8Value constructors

[pinepain]

After we update v8 dependency to 6.3, we have to implement this change.

Related commits in v8:

• add isolate argument to constructor: https://github.com/v8/v8/commit/fe598532ec1317e8b85343133be9fb708e07bd2e
• deprecate non-Isolate constructors: https://github.com/v8/v8/commit/ce05578aa6606cf662984a968cc99d421cea30d8