Fix issues when stressing compaction with WeakMaps.

src/mark-compact.cc

Index: src/mark-compact.cc
diff --git a/src/mark-compact.cc b/src/mark-compact.cc
index 0f6a5663fbdc5a683987f9c92d306b2609286aa7..29ed215e75547bd7b55f6820320a3440c3f6d829 100644
--- a/src/mark-compact.cc
+++ b/src/mark-compact.cc
@@ -1152,9 +1152,10 @@ class StaticMarkingVisitor : public StaticVisitorBase {
     JSWeakMap* weak_map = reinterpret_cast<JSWeakMap*>(object);
 
     // Enqueue weak map in linked list of encountered weak maps.
-    ASSERT(weak_map->next() == Smi::FromInt(0));
-    weak_map->set_next(collector->encountered_weak_maps());
-    collector->set_encountered_weak_maps(weak_map);
+    if (weak_map->next() == Smi::FromInt(0)) {
+      weak_map->set_next(collector->encountered_weak_maps());
+      collector->set_encountered_weak_maps(weak_map);
+    }
 
     // Skip visiting the backing hash table containing the mappings.
     int object_size = JSWeakMap::BodyDescriptor::SizeOf(map, object);
@@ -1170,9 +1171,15 @@ class StaticMarkingVisitor : public StaticVisitorBase {
         object_size);
 
     // Mark the backing hash table without pushing it on the marking stack.
-    ObjectHashTable* table = ObjectHashTable::cast(weak_map->table());
-    ASSERT(!MarkCompactCollector::IsMarked(table));
-    collector->SetMark(table, Marking::MarkBitFrom(table));
+    Object* table_object = weak_map->table();
+    if (!table_object->IsHashTable()) return;
+    ObjectHashTable* table = ObjectHashTable::cast(table_object);
+    Object** table_slot =
+        HeapObject::RawField(weak_map, JSWeakMap::kTableOffset);
+    MarkBit table_mark = Marking::MarkBitFrom(table);
+    collector->RecordSlot(table_slot, table_slot, table);
+    if (!table_mark.Get()) collector->SetMark(table, table_mark);
+    // Recording the map slot can be skipped, because maps are not compacted.
     collector->MarkObject(table->map(), Marking::MarkBitFrom(table->map()));
     ASSERT(MarkCompactCollector::IsMarked(table->map()));
   }