Implement handlified String::Flatten.

src/json-stringifier.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 287 matching lines @@
   static const int kJsonQuoteWorstCaseBlowup = 6;
   static const int kSpaceForQuotes = 2;
   int worst_case_length =
       object->length() * kJsonQuoteWorstCaseBlowup + kSpaceForQuotes;
 
   if (worst_case_length > 32 * KB) {  // Slow path if too large.
     BasicJsonStringifier stringifier(isolate);
     return stringifier.Stringify(object);
   }
 
-  FlattenString(object);
+  object = String::Flatten(object);
   ASSERT(object->IsFlat());
   if (object->IsOneByteRepresentationUnderneath()) {
     Handle<String> result = isolate->factory()->NewRawOneByteString(
         worst_case_length).ToHandleChecked();
     DisallowHeapAllocation no_gc;
     return StringifyString_<SeqOneByteString>(
         isolate,
         object->GetFlatContent().ToOneByteVector(),
         result);
   } else {
@@ skipping 547 matching lines @@
 
 template <>
 Vector<const uc16> BasicJsonStringifier::GetCharVector(Handle<String> string) {
   String::FlatContent flat = string->GetFlatContent();
   ASSERT(flat.IsTwoByte());
   return flat.ToUC16Vector();
 }
 
 
 void BasicJsonStringifier::SerializeString(Handle<String> object) {
-  object = FlattenGetString(object);
+  object = String::Flatten(object);
   if (is_ascii_) {
     if (object->IsOneByteRepresentationUnderneath()) {
       SerializeString_<true, uint8_t>(object);
     } else {
       ChangeEncoding();
       SerializeString(object);
     }
   } else {
     if (object->IsOneByteRepresentationUnderneath()) {
       SerializeString_<false, uint8_t>(object);
     } else {
       SerializeString_<false, uc16>(object);
     }
   }
 }
 
 } }  // namespace v8::internal
 
 #endif  // V8_JSON_STRINGIFIER_H_