Revert of Add CHECKs to verify that we never finalize stale copies of external strings

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

Committed: https://crrev.com/6a63a6d4cf652667963c5fdf35e10eee399fcd61
Cr-Commit-Position: refs/heads/master@{#28932}

[jochen (gone - plz use gerrit), 2015-06-11 09:37:11]

Created Revert of Add CHECKs to verify that we never finalize stale copies of
external strings

[commit-bot: I haz the power, 2015-06-11 09:37:26]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1178703008/1

[commit-bot: I haz the power, 2015-06-11 09:37:35]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-06-11 09:37:56]

Patchset 1 (id:??) landed as
https://crrev.com/6a63a6d4cf652667963c5fdf35e10eee399fcd61
Cr-Commit-Position: refs/heads/master@{#28932}