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

src/value-serializer.h

Index: src/value-serializer.h
diff --git a/src/value-serializer.h b/src/value-serializer.h
index 93fc15166d4d0b375039b6e548d2c324011c8950..6e70731be8a4d67b921e9b7cc7befeeec29eb9bf 100644
--- a/src/value-serializer.h
+++ b/src/value-serializer.h
@@ -88,6 +88,7 @@ class ValueSerializer {
   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;
@@ -102,7 +103,7 @@ class ValueSerializer {
    * 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_;