The generational barrier for source objects records the entire source
object to be processed later during remembered set visitation. It's
planned to be used for Blink backing stores when an inlined object (or a
range thereof) is added (HeapAllocator::NotifyNewObject(s)).

An alternative approach would be to eagerly process the inlined objects
using a custom callback. However, this requires changing Visitors to
bring slots into the context. This approach should better work for
scenarios where small ranges or single elements are added, to avoid
processing potentially large backing stores. The followup CL implements
this idea.

Bug: chromium:1029379
Change-Id: Iacb59e4b10a66354526ed293d7f43f14d8761a8f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3460402Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79073}