TaskScheduler: No BACKGROUND threads when unsupported.

base/task_scheduler/scheduler_worker.h

Index: base/task_scheduler/scheduler_worker.h
diff --git a/base/task_scheduler/scheduler_worker.h b/base/task_scheduler/scheduler_worker.h
index 71d4cbcc91667b27be3537c8519aafc802bd4221..abdbb9fa2efc20f6ea73bb18df2a4f41779fce13 100644
--- a/base/task_scheduler/scheduler_worker.h
+++ b/base/task_scheduler/scheduler_worker.h
@@ -77,11 +77,13 @@ class BASE_EXPORT SchedulerWorker {
 
   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,