Instead of writing the forwarding pointer of this object and treating it like
an object that would survive on scavenge just write the actual string pointer
to the outer slot. As a consequence, the ThingString will not look like a live
object and is handled properly when pruning the external string table.

Bug: v8:8249
Test: test/cctest/heap/test-external-string-tracker.cc
Change-Id: I975900213e2e4b598f298c8f78b6c6047c9e6da4
Reviewed-on: https://chromium-review.googlesource.com/1252885Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56311}