[serializer] encode recent long-encoded root list items as hot objects.

src/snapshot/code-serializer.cc

Index: src/snapshot/code-serializer.cc
diff --git a/src/snapshot/code-serializer.cc b/src/snapshot/code-serializer.cc
index 8115090b0a39d8112e809ebc88384463a18480c2..4229607c75a2173664ea038bedc0c42a6f4777c3 100644
--- a/src/snapshot/code-serializer.cc
+++ b/src/snapshot/code-serializer.cc
@@ -47,13 +47,15 @@ ScriptData* CodeSerializer::Serialize(Isolate* isolate,
 
 void CodeSerializer::SerializeObject(HeapObject* obj, HowToCode how_to_code,
                                      WhereToPoint where_to_point, int skip) {
+  if (SerializeHotObject(obj, how_to_code, where_to_point, skip)) return;
+
   int root_index = root_index_map_.Lookup(obj);
   if (root_index != RootIndexMap::kInvalidRootIndex) {
     PutRoot(root_index, obj, how_to_code, where_to_point, skip);
     return;
   }
 
-  if (SerializeKnownObject(obj, how_to_code, where_to_point, skip)) return;
+  if (SerializeBackReference(obj, how_to_code, where_to_point, skip)) return;
 
   FlushSkip(skip);