Adds a SHUTDOWN_TASK_QUEUE and a PreShutdown api to the scheduler.

components/scheduler/child/child_scheduler.h

 // Copyright 2015 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 COMPONENTS_SCHEDULER_CHILD_CHILD_SCHEDULER_H_
 #define COMPONENTS_SCHEDULER_CHILD_CHILD_SCHEDULER_H_
 
 #include "base/message_loop/message_loop.h"
 #include "components/scheduler/child/single_thread_idle_task_runner.h"
 #include "components/scheduler/scheduler_export.h"
 
 namespace base {
 class MessageLoop;
 }
 
 namespace scheduler {
 
 class SCHEDULER_EXPORT ChildScheduler {
  public:
   virtual ~ChildScheduler() {}
 
   // Returns the default task runner.
   virtual scoped_refptr<base::SingleThreadTaskRunner> DefaultTaskRunner() = 0;
 
   // Returns the idle task runner. Tasks posted to this runner may be reordered
   // relative to other task types and may be starved for an arbitrarily long
   // time if no idle time is available.
   virtual scoped_refptr<SingleThreadIdleTaskRunner> IdleTaskRunner() = 0;
 
+  // Returns the shutdown task runner. Intended for tasks that must be able to
+  // run even after PreShutdown() is called.
+  virtual scoped_refptr<base::SingleThreadTaskRunner> ShutdownTaskRunner() = 0;
+
   // Returns true if there is high priority work pending on the main thread
   // and the caller should yield to let the scheduler service that work. Note
   // that this is a stricter condition than |IsHighPriorityWorkAnticipated|,
   // restricted to the case where real work is pending.
   // Must be called from the thread this scheduler was created on.
   virtual bool ShouldYieldForHighPriorityWork() = 0;
 
   // Returns true if a currently running idle task could exceed its deadline
   // without impacting user experience too much. This should only be used if
   // there is a task which cannot be pre-empted and is likely to take longer
   // than the largest expected idle task deadline. It should NOT be polled to
   // check whether more work can be performed on the current idle task after
   // its deadline has expired - post a new idle task for the continuation of the
   // work in this case.
   // Must be called from the thread this scheduler was created on.
   virtual bool CanExceedIdleDeadlineIfRequired() const = 0;
 
   // Adds or removes a task observer from the scheduler. The observer will be
   // notified before and after every executed task. These functions can only be
   // called on the thread this scheduler was created on.
   virtual void AddTaskObserver(
       base::MessageLoop::TaskObserver* task_observer) = 0;
   virtual void RemoveTaskObserver(
       base::MessageLoop::TaskObserver* task_observer) = 0;
 
+  // Optional. This stops all queues except for the shutdown queue. Must be
+  // called from the thread this scheduler was created on.
+  virtual void PreShutdown() = 0;
+
   // Shuts down the scheduler by dropping any remaining pending work in the work
   // queues. After this call any work posted to the task runners will be
   // silently dropped.
   virtual void Shutdown() = 0;
 
  protected:
   ChildScheduler() {}
   DISALLOW_COPY_AND_ASSIGN(ChildScheduler);
 };
 
 }  // namespace scheduler
 
 #endif  // COMPONENTS_SCHEDULER_CHILD_CHILD_SCHEDULER_H_