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

test/mjsunit/es6/symbols.js

Index: test/mjsunit/es6/symbols.js
diff --git a/test/mjsunit/es6/symbols.js b/test/mjsunit/es6/symbols.js
index 9bac41f863ad11d70c6443b5e7f88369fc53f9e4..a21afb3770443e044882788549a830bc6ac8bdd7 100644
--- a/test/mjsunit/es6/symbols.js
+++ b/test/mjsunit/es6/symbols.js
@@ -555,7 +555,9 @@ 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"));