src: use HeapStatistics to get external memory

V8 is improving the way external memory is internally managed and how
it is reported.
External memory should now be retrieved using HeapStatistics.

Refs: https://github.com/v8/node/pull/80

PR-URL: https://github.com/nodejs/node/pull/22754


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Ali Ijaz Sheikh <ofrobots@google.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>