[reference - do not submit] Always create four pools in TaskSchedulerImpl.

base/task_scheduler/scheduler_single_thread_task_runner_manager.h

 // Copyright 2017 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 BASE_TASK_SCHEDULER_SCHEDULER_SINGLE_THREAD_TASK_RUNNER_MANAGER_H_
 #define BASE_TASK_SCHEDULER_SCHEDULER_SINGLE_THREAD_TASK_RUNNER_MANAGER_H_
 
+#include <string>
 #include <vector>
 
 #include "base/atomicops.h"
 #include "base/base_export.h"
 #include "base/logging.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/task_scheduler/scheduler_lock.h"
-#include "base/task_scheduler/scheduler_worker_pool_params.h"
-#include "base/task_scheduler/task_scheduler.h"
+#include "base/threading/platform_thread.h"
 
 namespace base {
 
 class TaskTraits;
 class SingleThreadTaskRunner;
 
 namespace internal {
 
 class DelayedTaskManager;
 class SchedulerWorker;
 class TaskTracker;
 
 class BASE_EXPORT SchedulerSingleThreadTaskRunnerManager final {
  public:
   SchedulerSingleThreadTaskRunnerManager(
-      const std::vector<SchedulerWorkerPoolParams>& worker_pool_params_vector,
-      const TaskScheduler::WorkerPoolIndexForTraitsCallback&
-          worker_pool_index_for_traits_callback,
       TaskTracker* task_tracker,
       DelayedTaskManager* delayed_task_manager);
   ~SchedulerSingleThreadTaskRunnerManager();
 
+  // Create a SingleThreadTaskRunner which runs tasks with |traits| on a
+  // dedicated thread named "TaskScheduler" + |name| + "SingleThread" + index.
+  // |priority_hint| is the preferred thread priority; the actual thread
+  // priority depends on shutdown state and platform capabilities.
   scoped_refptr<SingleThreadTaskRunner> CreateSingleThreadTaskRunnerWithTraits(
+      const std::string& name,
+      ThreadPriority priority_hint,
       const TaskTraits& traits);
 
   void JoinForTesting();
 
  private:
   class SchedulerSingleThreadTaskRunner;
 
   SchedulerWorker* CreateAndRegisterSchedulerWorker(
-      const SchedulerWorkerPoolParams& params);
+      const std::string& name,
+      ThreadPriority priority_hint);
 
   void UnregisterSchedulerWorker(SchedulerWorker* worker);
 
-  const std::vector<SchedulerWorkerPoolParams> worker_pool_params_vector_;
-  const TaskScheduler::WorkerPoolIndexForTraitsCallback
-      worker_pool_index_for_traits_callback_;
   TaskTracker* const task_tracker_;
   DelayedTaskManager* const delayed_task_manager_;
 
   // Synchronizes access to |workers_| and |worker_id_|.
   SchedulerLock workers_lock_;
   std::vector<scoped_refptr<SchedulerWorker>> workers_;
   int next_worker_id_ = 0;
 
 #if DCHECK_IS_ON()
   subtle::Atomic32 workers_unregistered_during_join_ = 0;
 #endif
 
   DISALLOW_COPY_AND_ASSIGN(SchedulerSingleThreadTaskRunnerManager);
 };
 
 }  // namespace internal
 }  // namespace base
 
 #endif  // BASE_TASK_SCHEDULER_SCHEDULER_SINGLE_THREAD_TASK_RUNNER_MANAGER_H_