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

src/snapshot/partial-serializer.h

Index: src/snapshot/partial-serializer.h
diff --git a/src/snapshot/partial-serializer.h b/src/snapshot/partial-serializer.h
index 9ce5cb31658d2a3acea683cc41a9cb076b689e63..0bf61dd05558b365dfae733da6d9cbca8e23d838 100644
--- a/src/snapshot/partial-serializer.h
+++ b/src/snapshot/partial-serializer.h
@@ -52,6 +52,7 @@ class PartialSerializer : public Serializer {
   Serializer* startup_serializer_;
   Object* global_object_;
   PartialCacheIndexMap partial_cache_index_map_;
+  int next_partial_cache_index_;
   DISALLOW_COPY_AND_ASSIGN(PartialSerializer);
 };