This field points to the start of the WASM memory buffer for the
instance, which is an ArrayBuffer and so 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:1218005
Change-Id: I847aebf5c29fcf1ab1163809350204db5b685a10
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/+/3359630Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Samuel Groß <saelo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78805}