Do not flush main thread tasks in ~ScopedTaskEnvironment.

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 37 matching lines @@
   const SchedulerWorkerPoolParams worker_pool_params(
       SchedulerWorkerPoolParams::StandbyThreadPolicy::ONE, kMaxThreads,
       kSuggestedReclaimTime);
   TaskScheduler::Create("ScopedTaskEnvironment");
   task_scheduler_ = TaskScheduler::GetInstance();
   TaskScheduler::GetInstance()->Start({worker_pool_params, worker_pool_params,
                                        worker_pool_params, worker_pool_params});
 }
 
 ScopedTaskEnvironment::~ScopedTaskEnvironment() {
-  RunLoop().RunUntilIdle();
-
+  // Intentionally do not RunLoop().RunUntilIdle() here as this simulates the
+  // 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);
 }
 
 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