src: extend `HeapStatistics` with new fields

src: Add does_zap_garbage, malloced_memory and
peak_malloced_memory to v8 HeapStatistics

Following https://github.com/nodejs/code-and-learn/issues/56 I
have exposed does_zap_garbage to HeapStatistics.
The other fields, malloced_memory and peak_malloced_memory don't
seem to be in the current version of v8 in master.

PR-URL: https://github.com/nodejs/node/pull/8610


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>