This commit updates the gen-postmortem-metadata.py script to
incorporate changes in V8 8.4. This removes the need to float a
patch to the script in Node.js.

Change-Id: I69da40e792f22748b0eee2952b9009b2f03d13f3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2565275Reviewed-by: Joyee Cheung <joyee@igalia.com>
Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Joyee Cheung <joyee@igalia.com>
Cr-Commit-Position: refs/heads/master@{#71646}