Do not flush main thread tasks in ~ScopedTaskEnvironment.

base/test/scoped_task_environment.h

 // 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.
 
 #ifndef BASE_TEST_SCOPED_TASK_ENVIRONMENT_H_
 #define BASE_TEST_SCOPED_TASK_ENVIRONMENT_H_
 
 #include "base/macros.h"
 #include "base/message_loop/message_loop.h"
 
@@ skipping 45 matching lines @@
     DEFAULT,
     // The main thread pumps UI messages.
     UI,
     // The main thread pumps asynchronous IO messages.
     IO,
   };
 
   ScopedTaskEnvironment(
       MainThreadType main_thread_type = MainThreadType::DEFAULT);
 
-  // Runs pending (Thread|Sequenced)TaskRunnerHandle tasks and pending
-  // BLOCK_SHUTDOWN TaskScheduler tasks. Then, unregisters the TaskScheduler and
-  // the (Thread|Sequenced)TaskRunnerHandle.
+  // Waits until no undelayed TaskScheduler tasks remain. Then, unregisters the
+  // TaskScheduler and the (Thread|Sequenced)TaskRunnerHandle.
   ~ScopedTaskEnvironment();
 
   // Synchronously runs (Thread|Sequenced)TaskRunnerHandle tasks until no
   // undelayed (Thread|Sequenced)TaskRunnerHandle or TaskScheduler tasks remain.
   void RunUntilIdle();
 
  private:
   // Note: |message_loop_| is an implementation detail and will be replaced in
   // the future, do NOT rely on the presence of a MessageLoop beyond
   // (Thread|Sequenced)TaskRunnerHandle and RunLoop.
   MessageLoop message_loop_;
 
   const TaskScheduler* task_scheduler_ = nullptr;
 
   DISALLOW_COPY_AND_ASSIGN(ScopedTaskEnvironment);
 };
 
 }  // namespace test
 }  // namespace base
 
 #endif  // BASE_TEST_SCOPED_ASYNC_TASK_SCHEDULER_H_