Do not take a MessageLoopForIO in the constructor of TaskTrackerPosix.

base/task_scheduler/task_tracker_posix.cc

Index: base/task_scheduler/task_tracker_posix.cc
diff --git a/base/task_scheduler/task_tracker_posix.cc b/base/task_scheduler/task_tracker_posix.cc
index 5c54f371949acf6100eda4f0d8a301057402d4e5..91fdec817a5d4e7463f05da2234a446a53636a9c 100644
--- a/base/task_scheduler/task_tracker_posix.cc
+++ b/base/task_scheduler/task_tracker_posix.cc
@@ -12,15 +12,17 @@
 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::Start(
+    MessageLoopForIO* watch_file_descriptor_message_loop) {
+  DCHECK(watch_file_descriptor_message_loop);
+  watch_file_descriptor_message_loop_ = watch_file_descriptor_message_loop_;
+}
+
 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));