Implement PostDelayedTaskAt for guaranteed timer ordering

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 8fd5359e8cfc38647aa567031a51f34384dcc526..14f9efb1caa72ea413850a8c86a25b5b1380624c 100644
--- a/components/scheduler/child/child_scheduler.h
+++ b/components/scheduler/child/child_scheduler.h
@@ -7,6 +7,7 @@
 
 #include "base/message_loop/message_loop.h"
 #include "components/scheduler/child/single_thread_idle_task_runner.h"
+#include "components/scheduler/child/task_queue.h"
 #include "components/scheduler/scheduler_export.h"
 
 namespace base {
@@ -20,7 +21,7 @@ class SCHEDULER_EXPORT ChildScheduler {
   virtual ~ChildScheduler() {}
 
   // Returns the default task runner.
-  virtual scoped_refptr<base::SingleThreadTaskRunner> DefaultTaskRunner() = 0;
+  virtual scoped_refptr<TaskQueue> 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