deps: V8: cherry-pick 5ebd6fcd from upstream

Original commit message:
  [heap] Lower external allocation limit when external memory shrinks.

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

  Review-Url: https://codereview.chromium.org/2921883002
  Cr-Commit-Position: refs/heads/master@{#45726}

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>