This reverts commit b3d09001.

Reason for revert: https://chromium-review.googlesource.com/c/v8/v8/+/2565511 has to be reverted, and this was a follow-up to that

Original change's description:
> [objects] Remove uncached internal external string type
>
> We shouldn't be creating those anymore since they are not thread-safe.
>
> Bug: v8:7790
> Change-Id: I4546d995fa32eb076c8dfe9d95301fad719c9e07
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2615347
> Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
> Reviewed-by: Leszek Swirski <leszeks@chromium.org>
> Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#72006}

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

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

Bug: v8:7790
Change-Id: I4eb1a6b8446fa602eeb5bf29fbf1fe57182cdbf3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2627605Reviewed-by: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72122}