[in-place weak refs] Fix overzealous DCHECK.

BUG=v8:7308,v8:7567

Change-Id: I031c11817686570c8fa5fa772691cd90c54d9ab2
Reviewed-on: https://chromium-review.googlesource.com/966029Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51992}

src/heap/concurrent-marking.cc

@@ -144,6 +144,7 @@ class ConcurrentMarkingVisitor final
     for (int i = 0; i < snapshot.number_of_slots(); i++) {
       Object** slot = snapshot.slot(i);
       Object* object = snapshot.value(i);
+      DCHECK(!Internals::HasWeakHeapObjectTag(object));
       if (!object->IsHeapObject()) continue;
       MarkObject(HeapObject::cast(object));
       MarkCompactCollector::RecordSlot(host, slot, object);
@@ -375,7 +376,6 @@ class ConcurrentMarkingVisitor final
       for (Object** p = start; p < end; p++) {
         Object* object = reinterpret_cast<Object*>(
             base::Relaxed_Load(reinterpret_cast<const base::AtomicWord*>(p)));
-        DCHECK(!Internals::HasWeakHeapObjectTag(object));
         slot_snapshot_->add(p, object);
       }
     }