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

src/api.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/api.h"
 
 #include <string.h>  // For memcpy, strlen.
 #ifdef V8_USE_ADDRESS_SANITIZER
 #include <sanitizer/asan_interface.h>
 #endif  // V8_USE_ADDRESS_SANITIZER
@@ skipping 2751 matching lines @@
   has_pending_exception = !ToLocal<Value>(maybe, &result);
   RETURN_ON_FAILED_EXECUTION(Value);
   RETURN_ESCAPED(result);
 }
 
 Local<Value> JSON::Parse(Local<String> json_string) {
   RETURN_TO_LOCAL_UNCHECKED(Parse(Local<Context>(), json_string), Value);
 }
 
 MaybeLocal<String> JSON::Stringify(Local<Context> context,
-                                   Local<Object> json_object) {
+                                   Local<Object> json_object,
+                                   Local<String> gap) {
   PREPARE_FOR_EXECUTION(context, JSON, Stringify, String);
   i::Handle<i::Object> object = Utils::OpenHandle(*json_object);
+  i::Handle<i::String> gap_string = gap.IsEmpty()
+                                        ? isolate->factory()->empty_string()
+                                        : Utils::OpenHandle(*gap);
   i::Handle<i::Object> maybe;
   has_pending_exception =
-      !i::Runtime::BasicJsonStringify(isolate, object).ToHandle(&maybe);
+      !i::Runtime::BasicJsonStringify(isolate, object, gap_string)
+           .ToHandle(&maybe);
   RETURN_ON_FAILED_EXECUTION(String);
   Local<String> result;
   has_pending_exception =
       !ToLocal<String>(i::Object::ToString(isolate, maybe), &result);
   RETURN_ON_FAILED_EXECUTION(String);
   RETURN_ESCAPED(result);
 }
 
 // --- D a t a ---
 
@@ skipping 6010 matching lines @@
   Address callback_address =
       reinterpret_cast<Address>(reinterpret_cast<intptr_t>(callback));
   VMState<EXTERNAL> state(isolate);
   ExternalCallbackScope call_scope(isolate, callback_address);
   callback(info);
 }
 
 
 }  // namespace internal
 }  // namespace v8