Custom callbacks assume that untraced pointers always point to valid,
not freed objects. They must make sure that upon callback completion no
UntracedMembers point to an unreachable object. This may not hold true
if a custom callback for an old object operates with a reference to a
young object that was freed on a minor collection cycle. To maintain
the mentioned invariant, the CL calls custom callbacks for old objects
on every minor collection cycle.

The alternative options could be:
1) Replacing all UntracedMembers with WeakMembers, since WeakMember
   supports tracing and the barrier.
2) Emitting the generational barrier for UntracedMember + tracing
   UntracedMember on minor collection cycles.
The first option requires changing multiple use sites and can bring some
performance regression. The second option requires changing the GC logic
and the semantics of UntracedMember.

Bug: chromium:1029379
Change-Id: I9bb89e4787daf05990feed374dceca940be7be63
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3472499Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79221}