Reland of [elements] Minor hardening and cleanup of concat (patchset #1 id:1 of https://codereview.chromium.org/1825363002/ )

Reason for revert:
Test disabled. See:
https://codereview.chromium.org/1830583002/

Original issue's description:
> Revert of [elements] Minor hardening and cleanup of concat (patchset #7 id:120001 of https://codereview.chromium.org/1812753004/ )
>
> Reason for revert:
> [Sheriff] Something seems to leak:
> https://build.chromium.org/p/client.v8/builders/V8%20Linux64%20ASAN/builds/10838
>
> I don't see the direct connection to this CL though...
>
> Original issue's description:
> > [elements] Minor hardening and cleanup of concat
> >
> > BUG=
> >
> > Committed: https://crrev.com/b98b3fbbe3dd14548cb356339f52403c07ef33f4
> > Cr-Commit-Position: refs/heads/master@{#35027}
>
> TBR=jkummerow@chromium.org,cbruni@chromium.org
> # Skipping CQ checks because original CL landed less than 1 days ago.
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=
>
> Committed: https://crrev.com/d30d861a0cb0da11efd3b30eb3d03fbe56b4ba23
> Cr-Commit-Position: refs/heads/master@{#35028}

TBR=jkummerow@chromium.org,cbruni@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
BUG=

Review URL: https://codereview.chromium.org/1827573003

Cr-Commit-Position: refs/heads/master@{#35038}