Adds a SHUTDOWN_TASK_QUEUE and a PreShutdown api to the scheduler.

components/scheduler/child/prioritizing_task_queue_selector.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef COMPONENTS_SCHEDULER_CHILD_PRIORITIZING_TASK_QUEUE_SELECTOR_H_
 #define COMPONENTS_SCHEDULER_CHILD_PRIORITIZING_TASK_QUEUE_SELECTOR_H_
 
 #include <set>
 
 #include "base/compiler_specific.h"
@@ skipping 23 matching lines @@
     // empty. They can be starved by the other queues.
     BEST_EFFORT_PRIORITY,
     // Must be the last entry.
     QUEUE_PRIORITY_COUNT,
     FIRST_QUEUE_PRIORITY = CONTROL_PRIORITY,
   };
 
   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.
   bool IsQueueEnabled(size_t queue_index) const;
 
   // TaskQueueSelector implementation:
   void RegisterWorkQueues(
       const std::vector<const base::TaskQueue*>& work_queues) override;
   bool SelectWorkQueueToService(size_t* out_queue_index) override;
   void AsValueInto(base::trace_event::TracedValue* state) const override;
@@ skipping 31 matching lines @@
   // Number of high priority tasks which can be run before a normal priority
   // task should be selected to prevent starvation.
   // TODO(rmcilroy): Check if this is a good value.
   static const size_t kMaxStarvationTasks = 5;
 
   base::ThreadChecker main_thread_checker_;
   std::vector<const base::TaskQueue*> work_queues_;
   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);
 };
 
 }  // namespace scheduler
 
 #endif  // COMPONENTS_SCHEDULER_CHILD_PRIORITIZING_TASK_QUEUE_SELECTOR_H_