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.

Depends on the Blink change: https://crrev.com/c/3116259

Bug: chromium:1242795, chromium:1243350
Change-Id: I8d76da30893c165e3946322b6d02f6ea2c8e529e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3114064
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76493}