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

test/mjsunit/es6/symbols.js

 // Copyright 2013 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 537 matching lines @@
   assertTrue(verifier(Symbol, TypeError()));
   assertTrue(verifier(rSymbol, rError()));
   assertFalse(verifier(Symbol, rError()));
   assertFalse(verifier(rSymbol, TypeError()));
 }
 TestContext();
 
 
 function TestStringify(expected, input) {
   assertEquals(expected, JSON.stringify(input));
-  assertEquals(expected, JSON.stringify(input, null, 0));
+  assertEquals(expected, JSON.stringify(input, (key, value) => value));
+  assertEquals(JSON.stringify(input, null, "="),
+               JSON.stringify(input, (key, value) => value, "="));
 }
 
 TestStringify(undefined, Symbol("a"));
 TestStringify('[{}]', [Object(Symbol())]);
 var symbol_wrapper = Object(Symbol("a"))
 TestStringify('{}', symbol_wrapper);
 symbol_wrapper.a = 1;
 TestStringify('{"a":1}', symbol_wrapper);