This reverts commit 6e621f84.

Reason for revert: Suspicion of GC stress failures like https://ci.chromium.org/p/v8/builders/ci/V8%20Linux%20-%20gc%20stress/30260

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}

TBR=ulan@chromium.org,leszeks@chromium.org,dinfuehr@chromium.org

Change-Id: I11c12e25702eb816cf616593d817a6ee3f691188
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:10315
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2451029Reviewed-by: Shu-yu Guo <syg@chromium.org>
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70326}