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/+/2448792Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70321}