Adds a flag to support "Virtual Time" to the blink scheduler

components/scheduler/child/scheduler_helper.cc

Index: components/scheduler/child/scheduler_helper.cc
diff --git a/components/scheduler/child/scheduler_helper.cc b/components/scheduler/child/scheduler_helper.cc
index e0d683e7222dcec667edfd342e3a5fa5cf83c08a..2dbdccf587a82ef32c5942d8549579b45936d8d1 100644
--- a/components/scheduler/child/scheduler_helper.cc
+++ b/components/scheduler/child/scheduler_helper.cc
@@ -34,7 +34,6 @@ SchedulerHelper::SchedulerHelper(
               .SetShouldNotifyObservers(false))),
       default_task_runner_(NewTaskQueue(TaskQueue::Spec("default_tq")
                                             .SetShouldMonitorQuiescence(true))),
-      time_source_(new base::DefaultTickClock),
       observer_(nullptr),
       tracing_category_(tracing_category),
       disabled_by_default_tracing_category_(
@@ -79,12 +78,6 @@ scoped_refptr<TaskQueue> SchedulerHelper::ControlAfterWakeUpTaskRunner() {
   return control_after_wakeup_task_runner_;
 }
 
-void SchedulerHelper::SetTimeSourceForTesting(
-    scoped_ptr<base::TickClock> time_source) {
-  CheckOnValidThread();
-  time_source_ = time_source.Pass();
-}
-
 void SchedulerHelper::SetWorkBatchSizeForTesting(size_t work_batch_size) {
   CheckOnValidThread();
   DCHECK(task_queue_manager_.get());
@@ -96,8 +89,8 @@ TaskQueueManager* SchedulerHelper::GetTaskQueueManagerForTesting() {
   return task_queue_manager_.get();
 }
 
-base::TimeTicks SchedulerHelper::Now() const {
-  return time_source_->NowTicks();
+base::TickClock* SchedulerHelper::tick_clock() const {
+  return task_queue_manager_->tick_clock();
 }
 
 base::TimeTicks SchedulerHelper::NextPendingDelayedTaskRunTime() const {