Do not take a MessageLoopForIO in the constructor of TaskTrackerPosix.

base/task_scheduler/task_tracker_posix_unittest.cc

 // 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.
 
 #include "base/task_scheduler/task_tracker_posix.h"
 
 #include <unistd.h>
 
 #include <utility>
 
@@ skipping 13 matching lines @@
 namespace internal {
 
 // Verify that TaskTrackerPosix runs a Task it receives.
 TEST(TaskSchedulerTaskTrackerPosixTest, RunTask) {
   MessageLoopForIO message_loop;
   bool did_run = false;
   auto task = MakeUnique<Task>(
       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()));
   EXPECT_TRUE(did_run);
 }
 
 // Verify that FileDescriptorWatcher::WatchReadable() can be called from a task
 // running in TaskTrackerPosix without a crash.
 TEST(TaskSchedulerTaskTrackerPosixTest, FileDescriptorWatcher) {
   MessageLoopForIO message_loop;
   int fds[2];
   ASSERT_EQ(0, pipe(fds));
   auto task = MakeUnique<Task>(
       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()));
 
   // Run the MessageLoop to allow the read watch to be registered and
   // unregistered. This prevents a memory leak.
   RunLoop().RunUntilIdle();
 
   EXPECT_EQ(0, IGNORE_EINTR(close(fds[0])));
   EXPECT_EQ(0, IGNORE_EINTR(close(fds[1])));
 }
 
 }  // namespace internal
 }  // namespace base