[serializer] print use count of external references.

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index 69bcf7d756ab369d0b9003bccd3f1161c33323f7..8eca750f268727784f14142db40342640ba187c1 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -559,6 +559,10 @@ StartupData SnapshotCreator::CreateBlob(
   }
   data->contexts_.Clear();
 
+#ifdef DEBUG
+  i::ExternalReferenceTable::instance(isolate)->ResetCount();
+#endif  // DEBUG
+
   i::StartupSerializer startup_serializer(isolate, function_code_handling);
   startup_serializer.SerializeStrongReferences();
 
@@ -572,6 +576,13 @@ StartupData SnapshotCreator::CreateBlob(
   }
 
   startup_serializer.SerializeWeakReferencesAndDeferred();
+
+#ifdef DEBUG
+  if (i::FLAG_external_reference_stats) {
+    i::ExternalReferenceTable::instance(isolate)->PrintCount();
+  }
+#endif  // DEBUG
+
   i::SnapshotData startup_snapshot(&startup_serializer);
   StartupData result =
       i::Snapshot::CreateSnapshotBlob(&startup_snapshot, &context_snapshots);