Serializer: clear next link in weak cells.

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index c508dd28db3b2d7d3784185563bd270497a9b1cd..819cc46e3ed03ae23f44dc0eadb6f531d2e67d2b 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -1925,6 +1925,14 @@ void WeakCell::set_next(Object* val, WriteBarrierMode mode) {
 }
 
 
+void WeakCell::clear_next(Heap* heap) {
+  set_next(heap->the_hole_value(), SKIP_WRITE_BARRIER);
+}
+
+
+bool WeakCell::next_cleared() { return next()->IsTheHole(); }
+
+
 int JSObject::GetHeaderSize() {
   InstanceType type = map()->instance_type();
   // Check for the most common kind of JavaScript object before