A background thread can register a callback that is guaranteed to be
invoked after each GC in a safepoint before background threads resume.
This will be allow the background compiler and parser to keep raw
pointers to frequently accessed objects and ensure that they are fixed
up after GC.

Note that the existing global GC epilogues are run after background
threads resume, so they are unsafe for background threads.

Change-Id: I1c782f912d63afc09c4982d393a6f3805a318962
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2675933
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72548}