Serializer: canonicalize cleared weak cells.

src/snapshot/serialize.cc

Index: src/snapshot/serialize.cc
diff --git a/src/snapshot/serialize.cc b/src/snapshot/serialize.cc
index b7c7dc1dd1c012264705860fd832e087910fc67e..713f245f668fb507e8ae5788c5ecfabe31619084 100644
--- a/src/snapshot/serialize.cc
+++ b/src/snapshot/serialize.cc
@@ -1092,6 +1092,12 @@ void Deserializer::ReadData(Object** current, Object** limit, int source_space,
         break;
       }
 
+      case kClearedWeakCell: {
+        Object* cleared_weak_cell = isolate_->heap()->cleared_weak_cell();
+        UnalignedCopy(current++, &cleared_weak_cell);
+        break;
+      }
+
       case kSynchronize:
         // If we get here then that indicates that you have a mismatch between
         // the number of GC roots when serializing and deserializing.
@@ -1462,6 +1468,11 @@ bool Serializer::BackReferenceIsAlreadyAllocated(BackReference reference) {
 
 bool Serializer::SerializeKnownObject(HeapObject* obj, HowToCode how_to_code,
                                       WhereToPoint where_to_point, int skip) {
+  if (obj->IsWeakCell() && WeakCell::cast(obj)->cleared()) {
+    FlushSkip(skip);
+    sink_->Put(kClearedWeakCell, "ClearedWeakCell");
+    return true;
+  }
   if (how_to_code == kPlain && where_to_point == kStartOfObject) {
     // Encode a reference to a hot object by its index in the working set.
     int index = hot_objects_.Find(obj);