[serializer] Add API to warm up startup snapshot with an additional script.

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"
 
@@ skipping 34 matching lines @@
 
   void SerializeObject(HeapObject* o, HowToCode how_to_code,
                        WhereToPoint where_to_point, int skip) override;
 
   int PartialSnapshotCacheIndex(HeapObject* o);
   bool ShouldBeInThePartialSnapshotCache(HeapObject* o);
 
   Serializer* startup_serializer_;
   Object* global_object_;
   PartialCacheIndexMap partial_cache_index_map_;
+  int next_partial_cache_index_;
   DISALLOW_COPY_AND_ASSIGN(PartialSerializer);
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_SNAPSHOT_PARTIAL_SERIALIZER_H_