Previously we checked whether a thread's pc IsPcProcessed before pushing
to the stack of (postponed) active_threads_.  This commit moves the
IsPcProcessed check and corresponding MarkPcProcessed call to when the
thread is actually processed, i.e. when it is popped from the
active_threads_ stack again.

This fixes two issues:
- Consider what used to happen in the following scenario:
1. An active thread t is postponed (e.g. because it is a fork) and
 pushed on active_threads_.  IsPcProcessed(t.pc) is false, so t is
 not discarded and does actually end up on active_threads_.
2. Some other thread s is executed, and at some point s.pc == t.pc,
 i.e. t.pc is marked as processed.
3. t is popped from active_threads_ for processing.

In 3 we don't want to continue execution of t: After all, its pc is
already marked as processed.  But because previously we only checked
for IsPcProcessed in step 1 before pushing to active_threads_, we used
to continue execution in 3.  I don't think this is a correctness
issue, but possibly a performance problem.  In any case, this commit
moves the IsPcProcessed check from 1 to 3 and so fixes this.
- After flushing blocked_threads_, we push them to active_threads_
again.  While doing so, we used to mark these thread's pcs as processed.
This meant that sometimes a (fork of a) high priority thread was
cancelled by the IsPcProcessed check even though its pc was only
marked as processed by a thread with lower priority during flushing.
We need it to be the other way round:  The low priority thread should
be cancelled after its pc is processed by a thread with higher
priority.
With this commit we don't MarkPcProcessed during flushing, it's
postponed to when we're actually processing.  This was a correctness
issue, and there's a new corresponding test case.


Bug: v8:10765
Change-Id: Ie12682cf3f8a04222d907edd8a3ad25baa69465a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2388112
Commit-Queue: Martin Bidlingmaier <mbid@google.com>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69668}