Adds a SHUTDOWN_TASK_QUEUE and a PreShutdown api to the scheduler.

components/scheduler/child/scheduler_helper.h

Index: components/scheduler/child/scheduler_helper.h
diff --git a/components/scheduler/child/scheduler_helper.h b/components/scheduler/child/scheduler_helper.h
index 3cb6997f8bae7e7a3560282276b64c58b18309f3..8cd0f5b96e253da657003af465b43d0e93cae743 100644
--- a/components/scheduler/child/scheduler_helper.h
+++ b/components/scheduler/child/scheduler_helper.h
@@ -61,6 +61,10 @@ class SCHEDULER_EXPORT SchedulerHelper {
   // time if no idle time is available.
   scoped_refptr<SingleThreadIdleTaskRunner> IdleTaskRunner();
 
+  // Returns the shutdown task runner. Intended for tasks that must be able to
+  // run even after PreShutdown() is called.
+  scoped_refptr<base::SingleThreadTaskRunner> ShutdownTaskRunner();
+
   // Returns the control task runner.  Tasks posted to this runner are executed
   // with the highest priority. Care must be taken to avoid starvation of other
   // task queues.
@@ -82,6 +86,10 @@ class SCHEDULER_EXPORT SchedulerHelper {
   void AddTaskObserver(base::MessageLoop::TaskObserver* task_observer);
   void RemoveTaskObserver(base::MessageLoop::TaskObserver* task_observer);
 
+  // Optional. This stops all queues except for the shutdown queue. Must be
+  // called from the thread this class was created on.
+  void PreShutdown();
+
   // 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.
@@ -94,6 +102,7 @@ class SCHEDULER_EXPORT SchedulerHelper {
   enum QueueId {
     DEFAULT_TASK_QUEUE,
     IDLE_TASK_QUEUE,
+    SHUTDOWN_TASK_QUEUE,
     CONTROL_TASK_QUEUE,
     CONTROL_TASK_AFTER_WAKEUP_QUEUE,
     // Must be the last entry.
@@ -195,8 +204,12 @@ class SCHEDULER_EXPORT SchedulerHelper {
   scoped_refptr<base::SingleThreadTaskRunner> control_task_runner_;
   scoped_refptr<base::SingleThreadTaskRunner> control_task_after_wakeup_runner_;
   scoped_refptr<base::SingleThreadTaskRunner> default_task_runner_;
+  scoped_refptr<base::SingleThreadTaskRunner> shutdown_task_runner_;
   scoped_refptr<SingleThreadIdleTaskRunner> idle_task_runner_;
 
+  size_t total_task_queue_count_;
+  bool in_preshutdown_;
+
   // A bitmap which controls the set of queues that are checked for quiescence
   // before triggering a long idle period.
   uint64 quiescence_monitored_task_queue_mask_;