lib,src: drop dependency on v8::Private::ForApi()

Said function requires that a v8::Context has been entered first,
introducing a chicken-and-egg problem when creating the first context.

PR-URL: https://github.com/nodejs/node/pull/7082


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>