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

src/runtime/runtime-json.cc

Index: src/runtime/runtime-json.cc
diff --git a/src/runtime/runtime-json.cc b/src/runtime/runtime-json.cc
index 72fc758ec83abd40fae6383134b88b2134f795e5..02f9a1657b2c960418b3a8ae6a2a1c5c61423b91 100644
--- a/src/runtime/runtime-json.cc
+++ b/src/runtime/runtime-json.cc
@@ -25,11 +25,12 @@ RUNTIME_FUNCTION(Runtime_QuoteJSONString) {
 
 RUNTIME_FUNCTION(Runtime_BasicJSONStringify) {
   HandleScope scope(isolate);
-  DCHECK(args.length() == 1);
+  DCHECK(args.length() == 2);
   CONVERT_ARG_HANDLE_CHECKED(Object, object, 0);
+  CONVERT_ARG_HANDLE_CHECKED(String, gap, 1);
   Handle<Object> result;
   ASSIGN_RETURN_FAILURE_ON_EXCEPTION(
-      isolate, result, Runtime::BasicJsonStringify(isolate, object));
+      isolate, result, Runtime::BasicJsonStringify(isolate, object, gap));
   return *result;
 }