This is a reland of 8c8e6b41

Timeouts seem unrelated.

Original change's description:
> [offthread] Serialize new-space objects into old space
>
> If an object made it into serialization, it probably deserves to be in
> old space when it is deserialized. This will also make off-thread
> deserialization simpler, as there is no off-thread new space.
>
> Bug: chromium:1075999
> Change-Id: Icabf2f0ae0a3e0205a1094dd0ffe675e69bd1d8e
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2184291
> Commit-Queue: Leszek Swirski <leszeks@chromium.org>
> Reviewed-by: Jakob Gruber <jgruber@chromium.org>
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#67606}

TBR=jgruber@chromium.org,ulan@chromium.org

Bug: chromium:1075999
Change-Id: Iac4e4eebeca1c343250269cdaad17e23645e9e2b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2184970Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67620}