When a NativeContext is being serialized, the NativeContext's
microtask_queue is set to nullptr as it is not included in the snapshot.
However, when the sandbox is enabled, this will only set the pointer in
the external pointer table to nullptr, but not the handle stored in the
object. This then causes the deserialized object to briefly be invalid,
before it's microtask queue handle is (re-)initialized. If a GC runs
during that timeframe, it will see an invalid external pointer handle,
which may cause DCHECK failures.
To fix this, this CL now introduces a generic mechanism for clearing and
restoring external pointer slots for serialization.

Bug: v8:13218
Change-Id: I03c8779bbec0a42a0b66687e76c951b1887e6122
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/+/3850294Reviewed-by: Jakob Linke <jgruber@chromium.org>
Commit-Queue: Samuel Groß <saelo@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82677}