v8::EmbedderHeapTracer::TracePrologue may call back into V8 during
StartMarking. In this case we expect that the write barriers are set up and
consistent, i.e., global flag matches page flag.

Blink calls back into V8 in a corner case where sweeping is finalized on
incremental marking start which may trigger resettting a V8 Value which may
trigger DescriptorArray re-shuffling.

Bug: chromium:940003
Change-Id: Ia15c798d0faaab802df1c3b569b5b6a323a4fe59
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1514492Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60145}