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.h

@@ -0,0 +1,36 @@
+// Copyright (c) 2018 GitHub, Inc.
+// Use of this source code is governed by the MIT license that can be
+// found in the LICENSE file.
+
+#ifndef ATOM_BROWSER_MICROTASKS_RUNNER_H_
+#define ATOM_BROWSER_MICROTASKS_RUNNER_H_
+
+#include "base/message_loop/message_loop.h"
+
+namespace v8 {
+class Isolate;
+}
+
+namespace atom {
+
+// Microtasks like promise resolution, are run at the end of the current
+// task. This class implements a task observer that runs tells v8 to run them.
+// Microtasks runner implementation is based on the EndOfTaskRunner in blink.
+// Node follows the kExplicit MicrotasksPolicy, and we do the same in browser
+// process. Hence, we need to have this task observer to flush the queued
+// microtasks.
+class MicrotasksRunner : public base::MessageLoop::TaskObserver {
+ public:
+  explicit MicrotasksRunner(v8::Isolate* isolate);
+
+  // base::MessageLoop::TaskObserver
+  void WillProcessTask(const base::PendingTask& pending_task) override;
+  void DidProcessTask(const base::PendingTask& pending_task) override;
+
+ private:
+  v8::Isolate* isolate_;
+};
+
+}  // namespace atom
+
+#endif  // ATOM_BROWSER_MICROTASKS_RUNNER_H_