This avoids a benign race in setting the raw pointer inside CTP
destructor by not emitting the write at all. The handle is destructed
which means that we only need to destroy any backing node but may
leave the handle untouched.

Drive-by:
- Add a few more docs.
- Make Clear() thread-safe.
- Make assignment of a sentinel pointer thread-safe.
- Make assignment of a nullptr thread-safe.

Bug: chromium:1242795
Change-Id: I0d9dafa31c298053e87ba1eb75f99fa6e33fa10b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3114134
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76455}