BACKGROUND_UNPARK and BACKGROUND_SAFEPOINT were introduced in
crrev.com/c/2704075 but the LAST_GENERAL_BACKGROUND_SCOPE was
not updated, so they were not tracked properly by GCTracer.

Bug: v8:12425
Change-Id: Ibbd7f5410088c45454b6e22af4e038e8ef0dd7fa
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3650598Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Nikolaos Papaspyrou <nikolaos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80613}