- Allow deserializer to add entries to the StringTable without
  causing a gc.

This is a reland of 868ed8ee

Original change's description:
> [runtime] Decrease StringTable shrink limit
>
> Given that we have not seen any regressions yet we're trying a more aggressive
> limit.
>
> Bug: chromium:818642, v8:5443
> Change-Id: Ic45001ed6c042fc31cbba0d417d5060d2de8fb3a
> Reviewed-on: https://chromium-review.googlesource.com/975126
> Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> Commit-Queue: Camillo Bruni <cbruni@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#52145}

Bug: chromium:818642, v8:5443
Change-Id: I051c6a79e59ec40cf87cab5bf06c4c449f8113d0
Reviewed-on: https://chromium-review.googlesource.com/975643
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52242}