HAS_PROGRESS_BAR is set after page initialization at which point all
flags are assumed to be immutable while a GC is running.

Separating out the progress bar from flags allows setting it lazily at
allocation time.

Bug: v8:11915
Change-Id: I48a877e0e80d583d7a0fadef2546fc70417806e7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3085268
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76382}