Main changes:

- Introduce a new broker data kind kBackgroundSerialized for objects
  that can be serialized in the background (when direct reads are on).
  (I'm planning to remove kPossiblyBackgroundSerialized in a followup,
  in favor of a dynamic choice of kSerialized or kBackgroundSerialized).
- Make PropertyCell use that new kind.
- Introduce a bottleneck in runtime code for changes to PropertyCells
  and make sure that a certain protocol is followed that allows
  concurrent reads from the background thread.
- Improve interface of PropertyCell in various ways.

Bug: v8:7790
Change-Id: If3d7926c3b894808811348b4b2bed153f5c06897
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2661462Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72586}