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..a5caf213985ea20bd986cafb70bea32d76891207 100644
--- a/base/task_scheduler/task_tracker.h
+++ b/base/task_scheduler/task_tracker.h
@@ -73,6 +73,11 @@ class BASE_EXPORT TaskTracker {
   // cannot be called after this.
   void SetHasShutdownStartedForTesting();
 
+ protected:
+  // Runs |task|. An override is expected to call its parent's implementation
+  // but is free to perform extra work before and after doing so.
+  virtual void PerformRunTask(std::unique_ptr<Task> task);
+
  private:
   class State;