During weak processing we remember weak callbacks for objects in the old
generation. We should check the young-gc flag and enable generational GC
before weak processing, as otherwise we would miss the callbacks and
forget to update the weak refs.

Bug: chromium:1029379
Change-Id: I72c98d4926b57c36af6cc503ce34712f67d50f42
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3616721Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80286}