This field points to the start of an ArrayBuffer backing store, which
is guaranteed to be located inside the sandbox if it is enabled. As
such, this simply turns the field into a sandboxed pointer field.

Bug: chromium:1342548
Change-Id: I5a76e23cfc83b2a04cd461def1cd04337ccf5cf7
Cq-Include-Trybots: luci.v8.try:v8_linux64_heap_sandbox_dbg_ng,v8_linux_arm64_sim_heap_sandbox_dbg_ng
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3749190Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Samuel Groß <saelo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81579}