These tests rely on dropping references to objects either explicitly ("o =
null;") or implicitly ("o goes out of scope") and then doing gc. It's essential
that we haven't already marked the WeakCell pointing to o and marked it alive
before dropping the reference.

BUG=v8:8179

Change-Id: Ie0b73f05c4baa937cf6f28325454ff9087a71a2c
Reviewed-on: https://chromium-review.googlesource.com/c/1306437Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57115}