This also fixes external string table update after scavenge and
the fast promotion mode.

Bug: chromium:852420
Change-Id: I5d2e1b585b8c74970047867aa587f928e116ed73
Reviewed-on: https://chromium-review.googlesource.com/c/1454604
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59379}