Adds a SHUTDOWN_TASK_QUEUE and a PreShutdown api to the scheduler.

components/scheduler/child/prioritizing_task_queue_selector.h

Index: components/scheduler/child/prioritizing_task_queue_selector.h
diff --git a/components/scheduler/child/prioritizing_task_queue_selector.h b/components/scheduler/child/prioritizing_task_queue_selector.h
index a9a63eb12bde3fc5f0b6976ecce44f7db93509ac..b42482b6d291b9ca67d2c3e78a3736066ecf133d 100644
--- a/components/scheduler/child/prioritizing_task_queue_selector.h
+++ b/components/scheduler/child/prioritizing_task_queue_selector.h
@@ -41,14 +41,20 @@ class SCHEDULER_EXPORT PrioritizingTaskQueueSelector
   PrioritizingTaskQueueSelector();
   ~PrioritizingTaskQueueSelector() override;
 
-  // Set the priority of |queue_index| to |priority|.
+  // Blocks future EnableQueue or DisableQueue calls.
+  void PreShutdown();
+
+  // Set the priority of |queue_index| to |priority|. If PreShutdown() has been
+  // called this does nothing.
   void SetQueuePriority(size_t queue_index, QueuePriority priority);
 
   // Enable the |queue_index| with a priority of |priority|. By default all
-  // queues are enabled with normal priority.
+  // queues are enabled with normal priority. If PreShutdown() has been called
+  // this does nothing.
   void EnableQueue(size_t queue_index, QueuePriority priority);
 
-  // Disable the |queue_index|.
+  // Disable the |queue_index|. If PreShutdown() has been called this does
+  // nothing.
   void DisableQueue(size_t queue_index);
 
   // Whether |queue_index| is enabled.
@@ -100,6 +106,7 @@ class SCHEDULER_EXPORT PrioritizingTaskQueueSelector
   std::set<size_t> queue_priorities_[QUEUE_PRIORITY_COUNT];
   size_t starvation_count_;
   Observer* task_queue_selector_observer_;  // NOT OWNED
+  bool pre_shutdown_;
   DISALLOW_COPY_AND_ASSIGN(PrioritizingTaskQueueSelector);
 };