As sandboxed pointers assume a constant sandbox size (they are
essentially n-bit offsets), it is no longer useful to be able to create
smaller sandboxes. This CL simplifies the sandbox initialization logic
accordingly and adds CHECKS to ensure a fixed-size sandbox is created.

Bug: v8:10391
Change-Id: I6541ab769001e60c0256d3a719f926128a0a20b0
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/+/3647684Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Samuel Groß <saelo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81720}