The CL does following:
1) Makes sure young generation works and tests pass;
2) Provides CollectGarbageInYoungGenerationForTesting() that is needed
   to support remaining tests in Blink;
3) Moved cppgc_enable_young_generation GN flag to v8.gni to refer to it
   from Blink;
4) Bails out from marking TracedReferences in UnifiedHeapMarkingState;
5) Disables (temporarily) prompt freeing for young generation;
6) Fixes remembered set visitation for nullptr|kSentinel slots.

Bug: chromium:1029379
Change-Id: I5165fa22c8a0eaa708ef7a35a9978cb12e1cb13e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3429202Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Auto-Submit: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78901}