Ensure that the off-thread pages' marking bits (including the page
headers) are correct, and synchronised correctly on merge.

Bug: chromium:1011762
Change-Id: I46c66fb35d49d39eb0da3513c869baf49c366706
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2110020
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66805}