This guards against concurrent usages of PageBackend::Lookup() from
HeapRegistry which can race with adding/removing pages.

This race only manifests in debug mode.

Change-Id: If34dbc255faeda085e522501ff2995693cd97b2e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3129702
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76590}