TaskScheduler: No BACKGROUND threads when unsupported.

base/task_scheduler/scheduler_worker_pool_params.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_SCHEDULER_WORKER_POOL_PARAMS_H_
 #define BASE_TASK_SCHEDULER_SCHEDULER_WORKER_POOL_PARAMS_H_
 
 #include <string>
 
 #include "base/macros.h"
 #include "base/threading/platform_thread.h"
 
 namespace base {
 
 class TimeDelta;
 
 class BASE_EXPORT SchedulerWorkerPoolParams final {
  public:
   enum class IORestriction {
     ALLOWED,
     DISALLOWED,
   };
 
   // Construct a scheduler worker pool parameter object that instructs a
   // scheduler worker pool to use the label |name| and create up to
-  // |max_threads| threads of priority |thread_priority|. |io_restriction|
-  // indicates whether Tasks on the scheduler worker pool are allowed to make
-  // I/O calls. |suggested_reclaim_time| sets a suggestion on when to reclaim
-  // idle threads. The worker pool is free to ignore this value for performance
-  // or correctness reasons.
-  SchedulerWorkerPoolParams(
-      const std::string& name,
-      ThreadPriority thread_priority,
-      IORestriction io_restriction,
-      int max_threads,
-      const TimeDelta& suggested_reclaim_time);
+  // |max_threads| threads. |priority_hint| is the preferred thread priority;
+  // the actual thread priority depends on shutdown state and platform
+  // capabilities. |io_restriction| indicates whether Tasks on the scheduler
+  // worker pool are allowed to make I/O calls. |suggested_reclaim_time| sets a
+  // suggestion on when to reclaim idle threads. The worker pool is free to
+  // ignore this value for performance or correctness reasons.
+  SchedulerWorkerPoolParams(const std::string& name,
+                            ThreadPriority priority_hint,
+                            IORestriction io_restriction,
+                            int max_threads,
+                            const TimeDelta& suggested_reclaim_time);
   SchedulerWorkerPoolParams(SchedulerWorkerPoolParams&& other);
   SchedulerWorkerPoolParams& operator=(SchedulerWorkerPoolParams&& other);
 
   // Name of the pool. Used to label the pool's threads.
   const std::string& name() const { return name_; }
 
-  // Priority of the pool's threads.
-  ThreadPriority thread_priority() const { return thread_priority_; }
+  // Preferred priority for the pool's threads.
+  ThreadPriority priority_hint() const { return priority_hint_; }
 
   // Whether I/O is allowed in the pool.
   IORestriction io_restriction() const { return io_restriction_; }
 
   // Maximum number of threads in the pool.
   size_t max_threads() const { return max_threads_; }
 
   // Suggested reclaim time for threads in the worker pool.
   const TimeDelta& suggested_reclaim_time() const {
     return suggested_reclaim_time_;
   }
 
  private:
   std::string name_;
-  ThreadPriority thread_priority_;
+  ThreadPriority priority_hint_;
   IORestriction io_restriction_;
   size_t max_threads_;
   TimeDelta suggested_reclaim_time_;
 
   DISALLOW_COPY_AND_ASSIGN(SchedulerWorkerPoolParams);
 };
 
 }  // namespace base
 
 #endif  // BASE_TASK_SCHEDULER_SCHEDULER_WORKER_POOL_PARAMS_H_