Do not take a MessageLoopForIO in the constructor of TaskTrackerPosix.

base/task_scheduler/task_scheduler_impl.h

Index: base/task_scheduler/task_scheduler_impl.h
diff --git a/base/task_scheduler/task_scheduler_impl.h b/base/task_scheduler/task_scheduler_impl.h
index 7791b4656db8af0430003161574271ddbcc453a6..3455eb4a705a613f1d8ac30473bf2ef1d11a37fb 100644
--- a/base/task_scheduler/task_scheduler_impl.h
+++ b/base/task_scheduler/task_scheduler_impl.h
@@ -18,10 +18,15 @@
 #include "base/task_scheduler/scheduler_worker_pool_impl.h"
 #include "base/task_scheduler/sequence.h"
 #include "base/task_scheduler/task_scheduler.h"
+#include "base/task_scheduler/task_tracker.h"
 #include "base/task_scheduler/task_traits.h"
 #include "base/threading/thread.h"
 #include "build/build_config.h"
 
+#if defined(OS_POSIX) && !defined(OS_NACL_SFI)
+#include "base/task_scheduler/task_tracker_posix.h"
+#endif
+
 namespace base {
 
 class HistogramBase;
@@ -29,7 +34,6 @@ class HistogramBase;
 namespace internal {
 
 class SchedulerSingleThreadTaskRunnerManager;
-class TaskTracker;
 
 // Default TaskScheduler implementation. This class is thread-safe.
 class BASE_EXPORT TaskSchedulerImpl : public TaskScheduler {
@@ -84,7 +88,11 @@ class BASE_EXPORT TaskSchedulerImpl : public TaskScheduler {
 
   const std::string name_;
   Thread service_thread_;
-  std::unique_ptr<TaskTracker> task_tracker_;
+#if defined(OS_POSIX) && !defined(OS_NACL_SFI)
+  TaskTrackerPosix task_tracker_;
+#else
+  TaskTracker task_tracker_;
+#endif
   DelayedTaskManager delayed_task_manager_;
   std::unique_ptr<SchedulerSingleThreadTaskRunnerManager>
       single_thread_task_runner_manager_;