TaskScheduler: Make the worker pools of TaskSchedulerImpl configurable

base/task_scheduler/task_scheduler.h

 // Copyright 2016 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_TASK_SCHEDULER_H_
 #define BASE_TASK_SCHEDULER_TASK_SCHEDULER_H_
 
 #include <memory>
 
 #include "base/base_export.h"
@@ skipping 30 matching lines @@
   // posted with the BLOCK_SHUTDOWN behavior will be run. When this returns:
   // - All SKIP_ON_SHUTDOWN tasks that were already running have completed their
   //   execution.
   // - All posted BLOCK_SHUTDOWN tasks have completed their execution.
   // - CONTINUE_ON_SHUTDOWN tasks might still be running.
   // Note that an implementation can keep threads and other resources alive to
   // support running CONTINUE_ON_SHUTDOWN after this returns. This can only be
   // called once.
   virtual void Shutdown() = 0;
 
-  // SetInstance() and InitializeDefaultTaskScheduler() register a TaskScheduler
-  // to handle tasks posted through the post_task.h API for this process. The
-  // registered TaskScheduler will only be deleted when a new TaskScheduler is
-  // registered (i.e. otherwise leaked on shutdown). The methods must not be
-  // called when TaskRunners created by the previous TaskScheduler are still
-  // alive. The methods are not thread-safe; proper synchronization is required
-  // to use the post_task.h API after registering a new TaskScheduler.
-
   // Registers |task_scheduler| to handle tasks posted through the post_task.h
-  // API for this process.
+  // API for this process. The registered TaskScheduler will only be deleted
+  // when a new TaskScheduler is registered (i.e. otherwise leaked on shutdown).
+  // This must not be called when TaskRunners created by the previous
+  // TaskScheduler are still alive. This method is not thread-safe; proper
+  // synchronization is required to use the post_task.h API after registering a
+  // new TaskScheduler.
   static void SetInstance(std::unique_ptr<TaskScheduler> task_scheduler);
 
-  // Initializes the default task scheduler for this process.
-  static void InitializeDefaultTaskScheduler();
-
-  // Retrieve the TaskScheduler set via SetInstance() or
-  // InitializeDefaultTaskScheduler(). This should be used very rarely; most
-  // users of TaskScheduler should use the post_task.h API.
+  // Retrieve the TaskScheduler set via SetInstance(). This should be used very
+  // rarely; most users of TaskScheduler should use the post_task.h API.
   static TaskScheduler* GetInstance();
 };
 
 }  // namespace base
 
 #endif  // BASE_TASK_SCHEDULER_TASK_SCHEDULER_H_