fix: add microtask runner and fix promise test (#15071)

* fix: Promise resolution and unit test

* Update to use microtask runner

* Address review

atom/browser/microtasks_runner.cc

@@ -0,0 +1,20 @@
+
+// Copyright (c) 2018 GitHub, Inc.
+// Use of this source code is governed by the MIT license that can be
+// found in the LICENSE file.
+
+#include "atom/browser/microtasks_runner.h"
+#include "v8/include/v8.h"
+
+namespace atom {
+
+MicrotasksRunner::MicrotasksRunner(v8::Isolate* isolate) : isolate_(isolate) {}
+
+void MicrotasksRunner::WillProcessTask(const base::PendingTask& pending_task) {}
+
+void MicrotasksRunner::DidProcessTask(const base::PendingTask& pending_task) {
+  v8::Isolate::Scope scope(isolate_);
+  v8::MicrotasksScope::PerformCheckpoint(isolate_);
+}
+
+}  // namespace atom