Implement PostDelayedTaskAt for guaranteed timer ordering

components/scheduler/child/null_worker_scheduler.cc

Index: components/scheduler/child/null_worker_scheduler.cc
diff --git a/components/scheduler/child/null_worker_scheduler.cc b/components/scheduler/child/null_worker_scheduler.cc
index 4abfb0e8e7cedd69acc0351b077c8c34147036e8..c80c28e8fc0649365686984df69a932220592961 100644
--- a/components/scheduler/child/null_worker_scheduler.cc
+++ b/components/scheduler/child/null_worker_scheduler.cc
@@ -8,19 +8,19 @@
 #include "base/message_loop/message_loop.h"
 #include "base/thread_task_runner_handle.h"
 #include "components/scheduler/child/null_idle_task_runner.h"
+#include "components/scheduler/child/null_task_queue.h"
 
 namespace scheduler {
 
 NullWorkerScheduler::NullWorkerScheduler()
-    : task_runner_(base::ThreadTaskRunnerHandle::Get()),
+    : task_runner_(new NullTaskQueue(base::ThreadTaskRunnerHandle::Get())),
       idle_task_runner_(new NullIdleTaskRunner()) {
 }
 
 NullWorkerScheduler::~NullWorkerScheduler() {
 }
 
-scoped_refptr<base::SingleThreadTaskRunner>
-NullWorkerScheduler::DefaultTaskRunner() {
+scoped_refptr<TaskQueue> NullWorkerScheduler::DefaultTaskRunner() {
   return task_runner_;
 }