scheduler: Return a base::SingleThreadTaskRunner for the default task runner.

components/scheduler/child/child_scheduler.h

Index: components/scheduler/child/child_scheduler.h
diff --git a/components/scheduler/child/child_scheduler.h b/components/scheduler/child/child_scheduler.h
index 5b582c8002b5e23213bdea73373aa2950c4a5eb1..87bcf56893bc8dbb50bb6895e1ebd597051f5d7f 100644
--- a/components/scheduler/child/child_scheduler.h
+++ b/components/scheduler/child/child_scheduler.h
@@ -22,8 +22,7 @@ class SCHEDULER_EXPORT ChildScheduler {
   virtual ~ChildScheduler() {}
 
   // Returns the default task runner.
-  // TODO(alexclarke): Change this to return a SingleThreadIdleTaskRunner.
-  virtual scoped_refptr<TaskQueue> DefaultTaskRunner() = 0;
+  virtual scoped_refptr<base::SingleThreadTaskRunner> DefaultTaskRunner() = 0;
 
   // Returns the idle task runner. Tasks posted to this runner may be reordered
   // relative to other task types and may be starved for an arbitrarily long