HeapRegistry requires a valid PageBackend for querying containment.
Reorder fields to guarantee that a backend is indeed available.

Bug: chromium:1056170
Change-Id: I32c0251d67ff99cd3609946154526f648c16c617
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2916818
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Auto-Submit: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74748}