This is a reland of 6e621f84

Original change's description:
> [heap] String::MakeThin can get away without NotifyObjectLayoutChange
>
> String::MakeThin doesn't need to invoke NotifyObjectLayoutChange because
> ThinString will only introduce tagged values and hence will not
> overwrite recorded slots with untagged values.
>
> Bug: v8:10315
> Change-Id: Iaff9c06cef763462eb57bf3debc5183ae8db6fa0
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2448792
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#70321}

The reland fixes the failures by first updating actual and only then
updating the map with a release store. This ensures that concurrent
markers will observe the pointer when visiting a ThinString and not some
stale data from before String::MakeThin.

Bug: v8:10315
Change-Id: I42db7ba9f44a48c920c997bf1258c561f3dd9bcc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2450066
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70346}