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

src/snapshot/serializer.h

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_SNAPSHOT_SERIALIZER_H_
 #define V8_SNAPSHOT_SERIALIZER_H_
 
 #include "src/isolate.h"
 #include "src/log.h"
 #include "src/objects.h"
@@ skipping 153 matching lines @@
   void PutSmi(Smi* smi);
 
   void PutBackReference(HeapObject* object, SerializerReference reference);
 
   void PutAttachedReference(SerializerReference reference,
                             HowToCode how_to_code, WhereToPoint where_to_point);
 
   // 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) {
       sink_.Put(kSkip, "SkipFromSerializeObject");
       sink_.PutInt(skip, "SkipDistanceFromSerializeObject");
     }
   }
 
   bool BackReferenceIsAlreadyAllocated(SerializerReference back_reference);
 
@@ skipping 129 matching lines @@
   SnapshotByteSink* sink_;
   int reference_representation_;
   int bytes_processed_so_far_;
   bool code_has_been_output_;
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_SNAPSHOT_SERIALIZER_H_