Fixes Object::SetAccessor to return false if the prop is unconfigurable.

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 4660 matching lines @@
   v8::Local<AccessorSignature> signature;
   auto info =
       MakeAccessorInfo(name, getter, setter, data, settings, attributes,
                        signature, i::FLAG_disable_old_api_accessors, false);
   if (info.is_null()) return Nothing<bool>();
   bool fast = obj->HasFastProperties();
   i::Handle<i::Object> result;
   has_pending_exception =
       !i::JSObject::SetAccessor(obj, info).ToHandle(&result);
   RETURN_ON_FAILED_EXECUTION_PRIMITIVE(bool);
-  if (result->IsUndefined(obj->GetIsolate())) return Nothing<bool>();
+  if (result->IsUndefined(obj->GetIsolate())) return Just(false);
   if (fast) {
     i::JSObject::MigrateSlowToFast(obj, 0, "APISetAccessor");
   }
   return Just(true);
 }
 
 
 Maybe<bool> Object::SetAccessor(Local<Context> context, Local<Name> name,
                                 AccessorNameGetterCallback getter,
                                 AccessorNameSetterCallback setter,
@@ skipping 5460 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