deps: V8: cherry-pick 502c6ae6 from upstream

Original commit message:
  [heap] Activate memory reducer on external memory activity.

  BUG=chromium:728228,chromium:626082
  CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_chromium_rel_ng

  Review-Url: https://codereview.chromium.org/2917853004
  Cr-Commit-Position: refs/heads/master@{#45671}

PR-URL: https://github.com/nodejs/node/pull/21269
Fixes: https://github.com/nodejs/node/issues/21021


Reviewed-By: Myles Borins <myles.borins@gmail.com>