Since external strings are used for things like source strings, we
should tenure them from creation.

Change-Id: I226ab9036836d76d8c17ed168ad97d7f0f824278
Reviewed-on: https://chromium-review.googlesource.com/1006961
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52677}