Support FileDescriptorWatcher in TaskScheduler.

base/task_scheduler/task_tracker.h

Index: base/task_scheduler/task_tracker.h
diff --git a/base/task_scheduler/task_tracker.h b/base/task_scheduler/task_tracker.h
index 8c19456e8772ed04407d5b078f0aad8753b45f37..ff24a2b5a8202d717f7f7b751e4e8848329babfa 100644
--- a/base/task_scheduler/task_tracker.h
+++ b/base/task_scheduler/task_tracker.h
@@ -73,6 +73,12 @@ class BASE_EXPORT TaskTracker {
   // cannot be called after this.
   void SetHasShutdownStartedForTesting();
 
+ protected:
+  // Runs |task|. An override should perform work before running the task,
+  // invoke the base class implementation to run the task and perform work after
+  // running the task.
+  virtual void PerformRunTask(std::unique_ptr<Task> task);
+
  private:
   class State;