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

src/snapshot/serializer.h

Index: src/snapshot/serializer.h
diff --git a/src/snapshot/serializer.h b/src/snapshot/serializer.h
index 46dd4a1401406af288f64f9a5c4da89351f1a3f3..45f891e190c53cc4c0c95ad34e42943fcc2caf1a 100644
--- a/src/snapshot/serializer.h
+++ b/src/snapshot/serializer.h
@@ -171,9 +171,13 @@ class Serializer : public SerializerDeserializer {
   // Emit alignment prefix if necessary, return required padding space in bytes.
   int PutAlignmentPrefix(HeapObject* object);
 
-  // Returns true if the object was successfully serialized.
-  bool SerializeKnownObject(HeapObject* obj, HowToCode how_to_code,
-                            WhereToPoint where_to_point, int skip);
+  // Returns true if the object was successfully serialized as hot object.
+  bool SerializeHotObject(HeapObject* obj, HowToCode how_to_code,
+                          WhereToPoint where_to_point, int skip);
+
+  // Returns true if the object was successfully serialized as back reference.
+  bool SerializeBackReference(HeapObject* obj, HowToCode how_to_code,
+                              WhereToPoint where_to_point, int skip);
 
   inline void FlushSkip(int skip) {
     if (skip != 0) {