The gc_in_progress flag was reset to false only after sweeping was done.
As a result, if we call CollectGarbage during an incremental GC and
after marking has finished, the we will observe that a gc is still in
progress but will not have a marker and crash.

The immediate solution is to move resetting the gc_in_progress flag such
that it indicates whether we didn't have the atomic pause yet. That
means we could have gc_in_progress==false and incremental sweeping still
running, which semantically negates the meaning of gc_in_progress.

Observing that gc_in_progress essentially becomes equivalent to having a
marker, this CL removes the gc_in_progress flag and replaces checks on
it with checks on marker.

Bug: chromium:1156170
Change-Id: Ic4b441ec248b5f7e222e988870e46d5166dd4dcc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2584875
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Auto-Submit: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71712}