cppgc: Fix data race in DCHECK between markers

Read of size 2 at 0x7eef001a3666 by main thread (mutexes: write M0):
 0: LoadEncoded
 1: IsMarked<(cppgc::internal::AccessMode)0>
 2: operator()
 3: DrainWorklistWithPredicate
 4: DrainWorklistWithBytesAndTimeDeadline

Previous atomic write of size 2 at 0x7eef001a3666 by thread T8:

 0: __cxx_atomic_compare_exchange_strong<unsigned short>
 1: compare_exchange_strong
 2: TryMarkAtomic
 3: MarkNoPush

Change-Id: I0708516382ea860c877ff76ee02216f6f27c9d04
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3858239
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Auto-Submit: Anton Bikineev <bikineev@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82776}

src/heap/cppgc/marker.cc

@@ -620,7 +620,7 @@ bool MarkerBase::ProcessWorklistsWithDeadline(
                 const HeapObjectHeader& header =
                     HeapObjectHeader::FromObject(item.base_object_payload);
                 DCHECK(!header.IsInConstruction<AccessMode::kNonAtomic>());
-                DCHECK(header.IsMarked<AccessMode::kNonAtomic>());
+                DCHECK(header.IsMarked<AccessMode::kAtomic>());
                 mutator_marking_state_.AccountMarkedBytes(header);
                 item.callback(&visitor(), item.base_object_payload);
               })) {