deps: remove extra field from v8::HeapStatistics

Remove the `_malloced_memory` field from the `HeapStatistics`
class to achieve full ABI compatibility with V8 5.0.

Ref: https://github.com/nodejs/node/pull/7016
PR-URL: https://github.com/nodejs/node/pull/7526


Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>