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

test/mjsunit/json.js

Index: test/mjsunit/json.js
diff --git a/test/mjsunit/json.js b/test/mjsunit/json.js
index 84f2056856a6526f245f22b8d05c8b2ef6c3733e..44548e4c13e90ac6e297eedae20b5d8064ae34c6 100644
--- a/test/mjsunit/json.js
+++ b/test/mjsunit/json.js
@@ -234,7 +234,9 @@ TestInvalid('"Garbage""After string"');
 
 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("true", true);
@@ -451,8 +453,8 @@ var counter = { get toJSON() { getCount++;
 // RegExps are not callable, so they are stringified as objects.
 TestStringify('{}', /regexp/);
 TestStringify('42', counter);
-assertEquals(2, getCount);
-assertEquals(2, callCount);
+assertEquals(4, getCount);
+assertEquals(4, callCount);
 
 var oddball2 = Object(42);
 var oddball3 = Object("foo");