Adds a SHUTDOWN_TASK_QUEUE and a PreShutdown api to the scheduler.

components/scheduler/child/child_scheduler.h

Index: components/scheduler/child/child_scheduler.h
diff --git a/components/scheduler/child/child_scheduler.h b/components/scheduler/child/child_scheduler.h
index 8fd5359e8cfc38647aa567031a51f34384dcc526..9f7df80d03d89e62e247ee8b1c88c1e1fb332756 100644
--- a/components/scheduler/child/child_scheduler.h
+++ b/components/scheduler/child/child_scheduler.h
@@ -27,6 +27,10 @@ class SCHEDULER_EXPORT ChildScheduler {
   // 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|,
@@ -52,6 +56,10 @@ class SCHEDULER_EXPORT ChildScheduler {
   virtual void RemoveTaskObserver(
       base::MessageLoop::TaskObserver* task_observer) = 0;
 
+  // Optional. This stops all queues except for shutdown and control queues.
+  // 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.