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..354656b71fb4779ff10a489bf4beca579a881178 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 +56,11 @@ class SCHEDULER_EXPORT ChildScheduler {
   virtual void RemoveTaskObserver(
       base::MessageLoop::TaskObserver* task_observer) = 0;
 
+  // Optional. This stops all queues except for shutdown and control queues.
+  // If posted from a different thread, it blocks until the thread this class
+  // was created on has completed the pre-shutdown.

[rmcilroy, 2015/04/24 09:41:02]

I'm not sure what the second sentence means. As far as I can tell this doesn't
block on a different thread?   It looks like this should only be called on the
main thread.

[alex clarke (OOO till 29th), 2015/04/24 14:10:05]

This comment was duplicated and I forgot to change this one, thanks.

+  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.