[serializer] introduce API to serialize internal fields

src/snapshot/partial-serializer.h

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_SNAPSHOT_PARTIAL_SERIALIZER_H_
 #define V8_SNAPSHOT_PARTIAL_SERIALIZER_H_
 
 #include "src/address-map.h"
 #include "src/snapshot/serializer.h"
 
 namespace v8 {
 namespace internal {
 
 class StartupSerializer;
 
 class PartialSerializer : public Serializer {
  public:
-  PartialSerializer(Isolate* isolate, StartupSerializer* startup_serializer);
+  PartialSerializer(Isolate* isolate, StartupSerializer* startup_serializer,
+                    v8::SerializeInternalFieldsCallback callback);
 
   ~PartialSerializer() override;
 
   // Serialize the objects reachable from a single object pointer.
   void Serialize(Object** o);
 
  private:
   void SerializeObject(HeapObject* o, HowToCode how_to_code,
                        WhereToPoint where_to_point, int skip) override;
 
   bool ShouldBeInThePartialSnapshotCache(HeapObject* o);
 
+  void SerializeInternalFields();
+
   StartupSerializer* startup_serializer_;
+  List<JSObject*> internal_field_holders_;
+  v8::SerializeInternalFieldsCallback serialize_internal_fields_;
   DISALLOW_COPY_AND_ASSIGN(PartialSerializer);
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_SNAPSHOT_PARTIAL_SERIALIZER_H_