The CL gets rid of the assumption that WeakCallbackItem::parameter
always points to an object with a custom callback, which might not be
the case for weak containers. This fixes failures in cppgc-non-default
bot.

Bug: chromium:1029379
Change-Id: I9ab6cb357153801efaa0d902c9eedff07d12ffe7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3490932Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79296}