Fix comment in scoped_task_environment.h.

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 15 matching lines @@
 // ScopedTaskEnvironment lives.
 //
 // Tasks posted through base/task_scheduler/post_task.h run on dedicated threads
 // as they are posted.
 //
 // Design and future improvements documented in
 // https://docs.google.com/document/d/1QabRo8c7D9LsYY3cEcaPQbOCLo8Tu-6VLykYXyl3Pkk/edit
 class ScopedTaskEnvironment {
  public:
   enum class MainThreadType {
-    // The main thread doesn't pump messages.
+    // The main thread doesn't pump system messages.
     DEFAULT,
     // The main thread pumps UI messages.
     UI,
     // The main thread pumps asynchronous IO messages.
     IO,
   };
 
   ScopedTaskEnvironment(
       MainThreadType main_thread_type = MainThreadType::DEFAULT);
 
@@ skipping 10 matching lines @@
 
   const TaskScheduler* task_scheduler_ = nullptr;
 
   DISALLOW_COPY_AND_ASSIGN(ScopedTaskEnvironment);
 };
 
 }  // namespace test
 }  // namespace base
 
 #endif  // BASE_TEST_SCOPED_ASYNC_TASK_SCHEDULER_H_