Do not take a MessageLoopForIO in the constructor of TaskTrackerPosix.

base/task_scheduler/task_tracker_posix_unittest.cc

Index: base/task_scheduler/task_tracker_posix_unittest.cc
diff --git a/base/task_scheduler/task_tracker_posix_unittest.cc b/base/task_scheduler/task_tracker_posix_unittest.cc
index b654a6d54380ab68b7d4fdc0fa50bc1c02242729..a6edc69863504d61d7b862905ea92af296abe27a 100644
--- a/base/task_scheduler/task_tracker_posix_unittest.cc
+++ b/base/task_scheduler/task_tracker_posix_unittest.cc
@@ -31,7 +31,8 @@ TEST(TaskSchedulerTaskTrackerPosixTest, RunTask) {
       FROM_HERE,
       Bind([](bool* did_run) { *did_run = true; }, Unretained(&did_run)),
       TaskTraits(), TimeDelta());
-  TaskTrackerPosix tracker(&message_loop);
+  TaskTrackerPosix tracker;
+  tracker.set_watch_file_descriptor_message_loop(&message_loop);
 
   EXPECT_TRUE(tracker.WillPostTask(task.get()));
   EXPECT_TRUE(tracker.RunTask(std::move(task), SequenceToken::Create()));
@@ -48,7 +49,8 @@ TEST(TaskSchedulerTaskTrackerPosixTest, FileDescriptorWatcher) {
       FROM_HERE, Bind(IgnoreResult(&FileDescriptorWatcher::WatchReadable),
                       fds[0], Bind(&DoNothing)),
       TaskTraits(), TimeDelta());
-  TaskTrackerPosix tracker(&message_loop);
+  TaskTrackerPosix tracker;
+  tracker.set_watch_file_descriptor_message_loop(&message_loop);
 
   EXPECT_TRUE(tracker.WillPostTask(task.get()));
   EXPECT_TRUE(tracker.RunTask(std::move(task), SequenceToken::Create()));