This reverts commit 3a6f75ac.

Reason for revert: performance regressions https://bugs.chromium.org/p/chromium/issues/detail?id=1163063

Original change's description:
> [objects] Remove MakeExternal case for uncached internal strings
>
> Concurrently accessing internal external uncached strings is not
> thread-safe. We are removing a case where we can make such a string
> through MakeExternal.
>
> Bug: v8:7790
> Change-Id: I958062c15cf40ccc330600bb572de98620866e54
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2565511
> Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
> Reviewed-by: Leszek Swirski <leszeks@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#71573}

TBR=leszeks@chromium.org,solanes@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: v8:7790
Change-Id: I5dcc734869c3c921eacd89426309141127a85f47
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2633547Reviewed-by: Santiago Aboy Solanes <solanes@chromium.org>
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72123}