Do not take a MessageLoopForIO in the constructor of TaskTrackerPosix.

base/task_scheduler/task_tracker_posix.h

Index: base/task_scheduler/task_tracker_posix.h
diff --git a/base/task_scheduler/task_tracker_posix.h b/base/task_scheduler/task_tracker_posix.h
index f957e4c0daf4d8b402a9b0ef413365b48955be90..53859b21ab63654d9ef7d7a7e68580acd4558b0b 100644
--- a/base/task_scheduler/task_tracker_posix.h
+++ b/base/task_scheduler/task_tracker_posix.h
@@ -21,18 +21,27 @@ 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);
 };