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

src/snapshot/deserializer.cc

Index: src/snapshot/deserializer.cc
diff --git a/src/snapshot/deserializer.cc b/src/snapshot/deserializer.cc
index 3f0008b3a88c26ce5f71a075ab98327c653afd08..ec755b596e8a99508828deff9127c4dfd3b3bc7d 100644
--- a/src/snapshot/deserializer.cc
+++ b/src/snapshot/deserializer.cc
@@ -77,6 +77,8 @@ void Deserializer::Deserialize(Isolate* isolate) {
   DCHECK_NULL(isolate_->thread_manager()->FirstThreadStateInUse());
   // No active handles.
   DCHECK(isolate_->handle_scope_implementer()->blocks()->is_empty());
+  // Partial snapshot cache is not yet populated.
+  DCHECK(isolate_->partial_snapshot_cache()->is_empty());
 
   {
     DisallowHeapAllocation no_gc;