No invalidation of slots necessary for String::MakeThin. ThinString
only stores tagged value, so it can't store an untagged value in a
recorded slot. CreateFillerObjectAt takes care of slots in case of
right-trimming objects.

Bug: v8:9454
Change-Id: Id16e8ebceb334a845bdbf77282fbeb2069efce7d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1794682Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63637}