TaskScheduler: No BACKGROUND threads when unsupported.

base/task_scheduler/scheduler_worker.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_H_
 #define BASE_TASK_SCHEDULER_SCHEDULER_WORKER_H_
 
 #include <memory>
 
 #include "base/base_export.h"
@@ skipping 59 matching lines @@
     // - The next WakeUp() could be more costly due to new thread creation.
     // - The worker will take this as a signal that it can detach, but it is not
     //   obligated to do so.
     // This MUST return false if SchedulerWorker::JoinForTesting() is in
     // progress.
     virtual bool CanDetach(SchedulerWorker* worker) = 0;
   };
 
   enum class InitialState { ALIVE, DETACHED };
 
-  // Creates a SchedulerWorker with priority |thread_priority| that runs Tasks
-  // from Sequences returned by |delegate|. |task_tracker| is used to handle
-  // shutdown behavior of Tasks. If |worker_state| is DETACHED, the thread will
-  // be created upon a WakeUp(). Returns nullptr if creating the underlying
-  // platform thread fails during Create().
+  // Creates a SchedulerWorker that runs Tasks from Sequences returned by
+  // |delegate|. |thread_priority| is the preferred thread priority; the actual
+  // thread priority depends on shutdown state and platform capabilities.
+  // |task_tracker| is used to handle shutdown behavior of Tasks. If
+  // |worker_state| is DETACHED, the thread will be created upon a WakeUp().
+  // Returns nullptr if creating the underlying platform thread fails during
+  // Create().
   static std::unique_ptr<SchedulerWorker> Create(
       ThreadPriority thread_priority,

[gab, 2016/08/03 13:35:24]

s/thread_priority/priority_hint/ or something?

[fdoray, 2016/08/03 18:42:49]

Done.

       std::unique_ptr<Delegate> delegate,
       TaskTracker* task_tracker,
       InitialState initial_state);
 
   // Destroying a SchedulerWorker in production is not allowed; it is always
   // leaked. In tests, it can only be destroyed after JoinForTesting() has
   // returned.
   ~SchedulerWorker();
 
   // Wakes up this SchedulerWorker if it wasn't already awake. After this
@@ skipping 46 matching lines @@
   // True once JoinForTesting() has been called.
   bool should_exit_for_testing_ = false;
 
   DISALLOW_COPY_AND_ASSIGN(SchedulerWorker);
 };
 
 }  // namespace internal
 }  // namespace base
 
 #endif  // BASE_TASK_SCHEDULER_SCHEDULER_WORKER_H_