TaskScheduler: No BACKGROUND threads when unsupported.

base/task_scheduler/scheduler_worker_pool_impl.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_IMPL_H_
 #define BASE_TASK_SCHEDULER_SCHEDULER_WORKER_POOL_IMPL_H_
 
 #include <stddef.h>
 
 #include <memory>
@@ skipping 86 matching lines @@
   class SchedulerWorkerDelegateImpl;
 
   SchedulerWorkerPoolImpl(StringPiece name,
                           SchedulerWorkerPoolParams::IORestriction
                               io_restriction,
                           const TimeDelta& suggested_reclaim_time,
                           TaskTracker* task_tracker,
                           DelayedTaskManager* delayed_task_manager);
 
   bool Initialize(
-      ThreadPriority thread_priority,
+      ThreadPriority priority_hint,
       size_t max_threads,
       const ReEnqueueSequenceCallback& re_enqueue_sequence_callback);
 
   // Wakes up the last worker from this worker pool to go idle, if any.
   void WakeUpOneWorker();
 
   // Adds |worker| to |idle_workers_stack_|.
   void AddToIdleWorkersStack(SchedulerWorker* worker);
 
   // Peeks from |idle_workers_stack_|.
@@ skipping 56 matching lines @@
   TaskTracker* const task_tracker_;
   DelayedTaskManager* const delayed_task_manager_;
 
   DISALLOW_COPY_AND_ASSIGN(SchedulerWorkerPoolImpl);
 };
 
 }  // namespace internal
 }  // namespace base
 
 #endif  // BASE_TASK_SCHEDULER_SCHEDULER_WORKER_POOL_IMPL_H_