Revert of Add CHECKs to verify that we never finalize stale copies of external strings (patchset #1 id:1 of https://codereview.chromium.org/1160253010/)

Reason for revert:
not needed anymore

Original issue's description:
> Add CHECKs to verify that we never finalize stale copies of external strings
>
> BUG=none
> R=ulan@chromium.org
> LOG=n
>
> Committed: https://crrev.com/ca2f8d811ca08783e38ce5dd42d38a6b6a7b876f
> Cr-Commit-Position: refs/heads/master@{#28811}

TBR=ulan@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=none

Review URL: https://codereview.chromium.org/1178703008

Cr-Commit-Position: refs/heads/master@{#28932}