Do not take a MessageLoopForIO in the constructor of TaskTrackerPosix.

base/task_scheduler/task_tracker_posix.cc

 // Copyright 2016 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/task_scheduler/task_tracker_posix.h"
 
 #include <utility>
 
 #include "base/files/file_descriptor_watcher_posix.h"
 #include "base/logging.h"
 
 namespace base {
 namespace internal {
 
-TaskTrackerPosix::TaskTrackerPosix(
-    MessageLoopForIO* watch_file_descriptor_message_loop)
-    : watch_file_descriptor_message_loop_(watch_file_descriptor_message_loop) {
-  DCHECK(watch_file_descriptor_message_loop_);
-}
-
+TaskTrackerPosix::TaskTrackerPosix() = default;
 TaskTrackerPosix::~TaskTrackerPosix() = default;
 
 void TaskTrackerPosix::PerformRunTask(std::unique_ptr<Task> task) {
+  DCHECK(watch_file_descriptor_message_loop_);
   FileDescriptorWatcher file_descriptor_watcher(
       watch_file_descriptor_message_loop_);
   TaskTracker::PerformRunTask(std::move(task));
 }
 
 }  // namespace internal
 }  // namespace base