Add ScopedTaskEnvironment::GetMainThreadTaskRunner().

base/test/scoped_task_environment.cc

 // Copyright 2017 The Chromium 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 "base/test/scoped_task_environment.h"
 
 #include "base/run_loop.h"
 #include "base/task_scheduler/scheduler_worker_pool_params.h"
 #include "base/task_scheduler/task_scheduler.h"
 #include "base/time/time.h"
@@ skipping 48 matching lines @@
   // replaced base::~MessageLoop() behaviour better.
   DCHECK_EQ(TaskScheduler::GetInstance(), task_scheduler_);
   // Without FlushForTesting(), DeleteSoon() and ReleaseSoon() tasks could be
   // skipped, resulting in memory leaks.
   TaskScheduler::GetInstance()->FlushForTesting();
   TaskScheduler::GetInstance()->Shutdown();
   TaskScheduler::GetInstance()->JoinForTesting();
   TaskScheduler::SetInstance(nullptr);
 }
 
+scoped_refptr<base::SingleThreadTaskRunner>
+ScopedTaskEnvironment::GetMainThreadTaskRunner() {
+  return message_loop_.task_runner();
+}
+
 void ScopedTaskEnvironment::RunUntilIdle() {
   for (;;) {
     TaskScheduler::GetInstance()->FlushForTesting();
 
     TaskObserver task_observer;
     MessageLoop::current()->AddTaskObserver(&task_observer);
     RunLoop().RunUntilIdle();
     MessageLoop::current()->RemoveTaskObserver(&task_observer);
 
     if (!task_observer.ran_task())
       return;
   }
 }
 
 }  // namespace test
 }  // namespace base