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 7ace6944c42b3d8ab5d0403f4bdd28c2fb46f73d..f33c5c0b34d0f2b8f5c06ce597de92c082892c06 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 +86,11 @@ 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 shutdown and control queues.
+  // Also ends any idle periods. If posted from a different thread, it blocks
+  // until the thread this class was created on has completed the pre-shutdown.
+  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 +103,7 @@ 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.
@@ -175,6 +185,7 @@ class SCHEDULER_EXPORT SchedulerHelper {
 
   bool ShouldWaitForQuiescence();
   void EnableLongIdlePeriodAfterWakeup();
+  void PreShutdownInternal(base::WaitableEvent* completion);
 
   base::ThreadChecker thread_checker_;
   scoped_ptr<PrioritizingTaskQueueSelector> task_queue_selector_;
@@ -190,8 +201,11 @@ 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_;
+
   // 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_;