Support FileDescriptorWatcher in TaskScheduler.

base/task_scheduler/task_tracker.h

 // 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.
 
 #ifndef BASE_TASK_SCHEDULER_TASK_TRACKER_H_
 #define BASE_TASK_SCHEDULER_TASK_TRACKER_H_
 
 #include <memory>
 
 #include "base/atomicops.h"
@@ skipping 55 matching lines @@
   bool HasShutdownStarted() const;
 
   // Returns true if shutdown has completed (Shutdown() has returned).
   bool IsShutdownComplete() const;
 
   // Causes HasShutdownStarted() to return true. Unlike when Shutdown() returns,
   // IsShutdownComplete() won't return true after this returns. Shutdown()
   // 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.

[gab, 2016/10/21 16:10:48]

An override doesn't "have to" perform work IMO, how about:

// Runs |task|. An override is expected to call its parent's implementation but
is free to perform extra work before and after doing so.

[fdoray, 2016/10/27 13:06:50]

Done.

+  virtual void PerformRunTask(std::unique_ptr<Task> task);
+
  private:
   class State;
 
   void PerformShutdown();
 
   // Called before WillPostTask() informs the tracing system that a task has
   // been posted. Updates |num_tasks_blocking_shutdown_| if necessary and
   // returns true if the current shutdown state allows the task to be posted.
   bool BeforePostTask(TaskShutdownBehavior shutdown_behavior);
 
@@ skipping 45 matching lines @@
   // Number of BLOCK_SHUTDOWN tasks posted during shutdown.
   HistogramBase::Sample num_block_shutdown_tasks_posted_during_shutdown_ = 0;
 
   DISALLOW_COPY_AND_ASSIGN(TaskTracker);
 };
 
 }  // namespace internal
 }  // namespace base
 
 #endif  // BASE_TASK_SCHEDULER_TASK_TRACKER_H_