Take advantage of fast properties in ValueSerializer when JSObject has them.

src/value-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_VALUE_SERIALIZER_H_
 #define V8_VALUE_SERIALIZER_H_
 
 #include <cstdint>
 #include <vector>
 
@@ skipping 70 matching lines @@
   void WriteRawBytes(const void* source, size_t length);
   uint8_t* ReserveRawBytes(size_t bytes);
 
   // Writing V8 objects of various kinds.
   void WriteOddball(Oddball* oddball);
   void WriteSmi(Smi* smi);
   void WriteHeapNumber(HeapNumber* number);
   void WriteString(Handle<String> string);
   Maybe<bool> WriteJSReceiver(Handle<JSReceiver> receiver) WARN_UNUSED_RESULT;
   Maybe<bool> WriteJSObject(Handle<JSObject> object) WARN_UNUSED_RESULT;
+  Maybe<bool> WriteJSObjectSlow(Handle<JSObject> object) WARN_UNUSED_RESULT;
   Maybe<bool> WriteJSArray(Handle<JSArray> array) WARN_UNUSED_RESULT;
   void WriteJSDate(JSDate* date);
   Maybe<bool> WriteJSValue(Handle<JSValue> value) WARN_UNUSED_RESULT;
   void WriteJSRegExp(JSRegExp* regexp);
   Maybe<bool> WriteJSMap(Handle<JSMap> map) WARN_UNUSED_RESULT;
   Maybe<bool> WriteJSSet(Handle<JSSet> map) WARN_UNUSED_RESULT;
   Maybe<bool> WriteJSArrayBuffer(JSArrayBuffer* array_buffer);
   Maybe<bool> WriteJSArrayBufferView(JSArrayBufferView* array_buffer);
 
   /*
    * Reads the specified keys from the object and writes key-value pairs to the
    * buffer. Returns the number of keys actually written, which may be smaller
    * if some keys are not own properties when accessed.
    */
-  Maybe<uint32_t> WriteJSObjectProperties(
+  Maybe<uint32_t> WriteJSObjectPropertiesSlow(
       Handle<JSObject> object, Handle<FixedArray> keys) WARN_UNUSED_RESULT;
 
   Isolate* const isolate_;
   std::vector<uint8_t> buffer_;
   Zone zone_;
 
   // To avoid extra lookups in the identity map, ID+1 is actually stored in the
   // map (checking if the used identity is zero is the fast way of checking if
   // the entry is new).
   IdentityMap<uint32_t> id_map_;
@@ skipping 104 matching lines @@
   Handle<SeededNumberDictionary> id_map_;
   MaybeHandle<SeededNumberDictionary> array_buffer_transfer_map_;
 
   DISALLOW_COPY_AND_ASSIGN(ValueDeserializer);
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_VALUE_SERIALIZER_H_