For cross-thread handling we require the atomic marking pause to
provide an atomically consistent view of markbits and weak references.
This is ensured by locking the whole atomic pause from entering to
weak processing.

This CL move ProcessWeakness() into FinishMarking() which allows to
nicely scope the upcomming lock from EnterAtomicPause() to
LeaveAtomicPause(). The alternative is requiring the caller to ensure
proper locking which is harder than ensuring that the Marker is
consistent.

Bug: chromium:1056170
Change-Id: Ib6028a0d76fcf9422c4a0d422fec3d568f106bf2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2442620
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70259}