[json] implement JSON.stringify gap pre-processing in C++.

src/runtime/runtime-json.cc

 // Copyright 2014 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.
 
 #include "src/runtime/runtime-utils.h"
 
 #include "src/arguments.h"
 #include "src/char-predicates-inl.h"
 #include "src/isolate-inl.h"
 #include "src/json-parser.h"
@@ skipping 10 matching lines @@
   Handle<Object> result;
   ASSIGN_RETURN_FAILURE_ON_EXCEPTION(
       isolate, result, BasicJsonStringifier::StringifyString(isolate, string));
   return *result;
 }
 
 RUNTIME_FUNCTION(Runtime_BasicJSONStringify) {
   HandleScope scope(isolate);
   DCHECK(args.length() == 2);
   CONVERT_ARG_HANDLE_CHECKED(Object, object, 0);
-  CONVERT_ARG_HANDLE_CHECKED(String, gap, 1);
+  CONVERT_ARG_HANDLE_CHECKED(Object, gap, 1);
   Handle<Object> result;
   ASSIGN_RETURN_FAILURE_ON_EXCEPTION(
-      isolate, result, BasicJsonStringifier(isolate, gap).Stringify(object));
+      isolate, result, BasicJsonStringifier(isolate).Stringify(object, gap));
   return *result;
 }
 
 RUNTIME_FUNCTION(Runtime_ParseJson) {
   HandleScope scope(isolate);
   DCHECK_EQ(1, args.length());
   CONVERT_ARG_HANDLE_CHECKED(Object, object, 0);
   Handle<String> source;
   ASSIGN_RETURN_FAILURE_ON_EXCEPTION(isolate, source,
                                      Object::ToString(isolate, object));
   source = String::Flatten(source);
   // Optimized fast case where we only have Latin1 characters.
   Handle<Object> result;
   ASSIGN_RETURN_FAILURE_ON_EXCEPTION(isolate, result,
                                      source->IsSeqOneByteString()
                                          ? JsonParser<true>::Parse(source)
                                          : JsonParser<false>::Parse(source));
   return *result;
 }
 
 }  // namespace internal
 }  // namespace v8