build: fix v8_enable_handle_zapping override

It was previously ignored by features.gypi and therefore
enabled by default for release builds.

See https://code.google.com/p/chromium/issues/detail?id=318206

PR-URL: https://github.com/nodejs/node/pull/2731


Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>