- Process the minidump in a separate function to avoid keeping
  references to the mmapped file during disposal
- Clear all MinidumpReader variables before disposing the mmapped file

Change-Id: I0ce468597329d6f7d703a08309e4be378d9c27cf
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3568469Reviewed-by: Liviu Rau <liviurau@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79750}