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..db56570229503dc16bc29b35d59a1ea4c680ae30 100644
--- a/base/task_scheduler/task_tracker_posix.h
+++ b/base/task_scheduler/task_tracker_posix.h
@@ -20,19 +20,23 @@ 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.
+// a task runs. Used on all POSIX platforms except NaCl SFI. Start() must be
+// called before the TaskTracker can run tasks.
 class BASE_EXPORT TaskTrackerPosix : public TaskTracker {
  public:
+  TaskTrackerPosix();
+  ~TaskTrackerPosix();
+
   // |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();
+  void Start(MessageLoopForIO* watch_file_descriptor_message_loop);
 
  private:
   // TaskTracker:
   void PerformRunTask(std::unique_ptr<Task> task) override;
 
-  MessageLoopForIO* const watch_file_descriptor_message_loop_;
+  // Set in Start() and cannot be read before Start().

[gab, 2017/04/12 19:17:34]

s/cannot/must not/

[fdoray, 2017/04/13 12:29:08]

n/a. superseded by comment about external synchronization.

+  MessageLoopForIO* watch_file_descriptor_message_loop_;
 
   DISALLOW_COPY_AND_ASSIGN(TaskTrackerPosix);
 };