Object shapes or sizes shouldn't change during the string fixup, but
we're seeing crashes that indicate that they might do anyway, so add
some more exhaustive checking to make sure they don't.

Bug: chromium:1086478
Change-Id: I36d41e036a32d8dd072000d900ba1900343d4608
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2214839
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68003}