[JSON] implement indentation in the BasicJsonStringifier and expose via API.

test/mjsunit/es6/proxies-json.js

 // 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 17 matching lines @@
 
 
 ///////////////////////////////////////////////////////////////////////////////
 // JSON.stringify
 
 
 function testStringify(expected, object) {
   // Test fast case that bails out to slow case.
   assertEquals(expected, JSON.stringify(object));
   // Test slow case.
-  assertEquals(expected, JSON.stringify(object, undefined, 0));
+  assertEquals(expected, JSON.stringify(object, (key, value) => value));
+  // Test gap.
+  assertEquals(JSON.stringify(object, null, "="),
+               JSON.stringify(object, (key, value) => value, "="));
 }
 
 
 // Test serializing a proxy, a function proxy, and objects that contain them.
 
 var handler1 = {
   get: function(target, name) {
     return name.toUpperCase();
   },
   ownKeys: function() {
@@ skipping 11 matching lines @@
 assertTrue(typeof(proxy_fun) === 'function');
 testStringify(undefined, proxy_fun);
 testStringify('[1,null]', [1, proxy_fun]);
 
 handler1.apply = function() { return 666; };
 testStringify(undefined, proxy_fun);
 testStringify('[1,null]', [1, proxy_fun]);
 
 var parent1a = { b: proxy1 };
 testStringify('{"b":{"a":"A","b":"B","c":"C"}}', parent1a);
+testStringify('{"b":{"a":"A","b":"B","c":"C"}}', parent1a);
 
 var parent1b = { a: 123, b: proxy1, c: true };
 testStringify('{"a":123,"b":{"a":"A","b":"B","c":"C"},"c":true}', parent1b);
 
 var parent1c = [123, proxy1, true];
 testStringify('[123,{"a":"A","b":"B","c":"C"},true]', parent1c);
 
 
 // Proxy with side effect.
 
@@ skipping 416 matching lines @@
 
 var result = JSON.parse('{"foo":0,"bar":1}', reviver);
 assertEquals({foo: 0, bar: proxy}, result);
 assertSame(result.bar, proxy);
 assertEquals(3, log.length);
 for (var i in log) assertSame(target, log[i][1]);
 
 assertEquals(["get", target, "length", proxy], log[0]);
 assertEquals(["get", target, "0", proxy], log[1]);
 assertEquals(["deleteProperty", target, "0"], log[2]);