buffer: fix ArrayBuffer checks

This commit fixes detection of ArrayBuffers from different V8 contexts.
This is especially a problem for environments like nw.js where the
node and browser V8 contexts are not shared.

PR-URL: https://github.com/nodejs/node/pull/8453


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>