Observers may start incremental marking and thus black allocation. Since
the object was allocated before invocing the observer the invariant
might not hold anymore.

Bug: v8:7174
Change-Id: I74fe1816303bedb28b69ec5ac83fd19e588acace
Reviewed-on: https://chromium-review.googlesource.com/813714Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49923}