Remove reinvented wheel for partial snapshot cache.

src/serialize.h

Index: src/serialize.h
diff --git a/src/serialize.h b/src/serialize.h
index b76abbcbac7e3895b2a8a8b3fcee9aa32ab2f64a..cfe9e1158dbad73e2c0779a955ae72c3e04b23c0 100644
--- a/src/serialize.h
+++ b/src/serialize.h
@@ -814,7 +814,7 @@ class StartupSerializer : public Serializer {
     // strong roots have been serialized we can create a partial snapshot
     // which will repopulate the cache with objects needed by that partial
     // snapshot.
-    isolate->set_serialize_partial_snapshot_cache_length(0);
+    isolate->partial_snapshot_cache()->Clear();
     InitializeCodeAddressMap();
   }