Previously it was implemented in api.cc, therefore requiring an additional
function call when accessing external pointer fields from embedder code with
the sandbox enabled. Now ReadExternalPointerField can be inlined.

Bug: v8:10391
Change-Id: Ia8cb2df148ac96f979fd3e22989b0ff6177abcec
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/+/3714245Reviewed-by: Igor Sheludko <ishell@chromium.org>
Auto-Submit: Samuel Groß <saelo@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81271}