Do not take a MessageLoopForIO in the constructor of TaskTrackerPosix.

base/task_scheduler/task_tracker_posix.h

 // 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.
 
 #ifndef BASE_TASK_SCHEDULER_TASK_TRACKER_POSIX_H_
 #define BASE_TASK_SCHEDULER_TASK_TRACKER_POSIX_H_
 
 #include <memory>
 
 #include "base/base_export.h"
 #include "base/macros.h"
 #include "base/task_scheduler/task_tracker.h"
 
 namespace base {
 
 class MessageLoopForIO;
 
 namespace internal {
 
 struct Task;
 
 // A TaskTracker that instantiates a FileDescriptorWatcher in the scope in which
 // a task runs. Used on all POSIX platforms except NaCl SFI.
+// set_watch_file_descriptor_message_loop() must be called before the
+// TaskTracker can run tasks.
 class BASE_EXPORT TaskTrackerPosix : public TaskTracker {
  public:
-  // |watch_file_descriptor_message_loop| is used to setup FileDescriptorWatcher
-  // in the scope in which a Task runs.
-  TaskTrackerPosix(MessageLoopForIO* watch_file_descriptor_message_loop);
+  TaskTrackerPosix();
   ~TaskTrackerPosix();
 
+  // Sets the MessageLoopForIO with which to setup FileDescriptorWatcher in the
+  // scope in which tasks run. Must be called before starting to run tasks.
+  // External synchronization is required between a call to this and a call to
+  // RunTask().
+  void set_watch_file_descriptor_message_loop(
+      MessageLoopForIO* watch_file_descriptor_message_loop) {
+    watch_file_descriptor_message_loop_ = watch_file_descriptor_message_loop;
+  }
+
  private:
   // TaskTracker:
   void PerformRunTask(std::unique_ptr<Task> task) override;
 
-  MessageLoopForIO* const watch_file_descriptor_message_loop_;
+  MessageLoopForIO* watch_file_descriptor_message_loop_ = nullptr;
 
   DISALLOW_COPY_AND_ASSIGN(TaskTrackerPosix);
 };
 
 }  // namespace internal
 }  // namespace base
 
 #endif  // BASE_TASK_SCHEDULER_TASK_TRACKER_POSIX_H_