Instead of alternating V8 and embedder steps and trying to find a
fixed point where both are empty, just run two phases and check
whether both have been observed as empty individually.

Bug: chromium:1044630, chromium:1049084
Change-Id: I7ffdca3086f1e7f772303272e370c4a6c1f83f24
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2041437
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66153}