Implement handlified String::Flatten.

src/json-parser.h

 // Copyright 2011 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 41 matching lines @@
  private:
   explicit JsonParser(Handle<String> source)
       : source_(source),
         source_length_(source->length()),
         isolate_(source->map()->GetHeap()->isolate()),
         factory_(isolate_->factory()),
         zone_(isolate_),
         object_constructor_(isolate_->native_context()->object_function(),
                             isolate_),
         position_(-1) {
-    FlattenString(source_);
+    source_ = String::Flatten(source_);
     pretenure_ = (source_length_ >= kPretenureTreshold) ? TENURED : NOT_TENURED;
 
     // Optimized fast case where we only have ASCII characters.
     if (seq_ascii) {
       seq_source_ = Handle<SeqOneByteString>::cast(source_);
     }
   }
 
   // Parse a string containing a single JSON value.
   MaybeHandle<Object> ParseJson();
@@ skipping 724 matching lines @@
 
   ASSERT_EQ('"', c0_);
   // Advance past the last '"'.
   AdvanceSkipWhitespace();
   return result;
 }
 
 } }  // namespace v8::internal
 
 #endif  // V8_JSON_PARSER_H_