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

components/scheduler/base/task_queue_manager.cc

Index: components/scheduler/base/task_queue_manager.cc
diff --git a/components/scheduler/base/task_queue_manager.cc b/components/scheduler/base/task_queue_manager.cc
index 099edd3f29082216b91a56fe839ffcf28041861e..64414b0466c86f8c9d461fcf9482591bf6c14397 100644
--- a/components/scheduler/base/task_queue_manager.cc
+++ b/components/scheduler/base/task_queue_manager.cc
@@ -8,7 +8,6 @@
 #include <set>
 
 #include "base/bind.h"
-#include "base/time/default_tick_clock.h"
 #include "components/scheduler/base/lazy_now.h"
 #include "components/scheduler/base/nestable_single_thread_task_runner.h"
 #include "components/scheduler/base/task_queue_impl.h"
@@ -30,7 +29,6 @@ TaskQueueManager::TaskQueueManager(
       task_was_run_on_quiescence_monitored_queue_(false),
       pending_dowork_count_(0),
       work_batch_size_(1),
-      time_source_(new base::DefaultTickClock),
       tracing_category_(tracing_category),
       disabled_by_default_tracing_category_(
           disabled_by_default_tracing_category),
@@ -175,7 +173,7 @@ void TaskQueueManager::UpdateWorkQueues(
   DCHECK(main_thread_checker_.CalledOnValidThread());
   TRACE_EVENT0(disabled_by_default_tracing_category_,
                "TaskQueueManager::UpdateWorkQueues");
-  internal::LazyNow lazy_now(this);
+  internal::LazyNow lazy_now(tick_clock());
 
   // Move any ready delayed tasks into the incomming queues.
   WakeupReadyDelayedQueues(&lazy_now);
@@ -196,7 +194,7 @@ void TaskQueueManager::UpdateWorkQueues(
 void TaskQueueManager::ScheduleDelayedWorkTask(
     scoped_refptr<internal::TaskQueueImpl> queue,
     base::TimeTicks delayed_run_time) {
-  internal::LazyNow lazy_now(this);
+  internal::LazyNow lazy_now(tick_clock());
   ScheduleDelayedWork(queue.get(), delayed_run_time, &lazy_now);
 }
 
@@ -232,7 +230,7 @@ void TaskQueueManager::DelayedDoWork() {
   DCHECK(main_thread_checker_.CalledOnValidThread());
 
   {
-    internal::LazyNow lazy_now(this);
+    internal::LazyNow lazy_now(tick_clock());
     WakeupReadyDelayedQueues(&lazy_now);
   }
 
@@ -319,8 +317,12 @@ void TaskQueueManager::DoWork(bool decrement_pending_dowork_count) {
   // TODO(alexclarke): Consider refactoring the above loop to terminate only
   // when there's no more work left to be done, rather than posting a
   // continuation task.
-  if (!selector_.EnabledWorkQueuesEmpty())
+  if (!selector_.EnabledWorkQueuesEmpty()) {
     MaybePostDoWorkOnMainRunner();
+  } else {
+    // Tell the task runner we have no more work.
+    main_task_runner_->OnNoMoreWork();
+  }
 }
 
 bool TaskQueueManager::SelectQueueToService(
@@ -415,20 +417,14 @@ void TaskQueueManager::RemoveTaskObserver(
   task_observers_.RemoveObserver(task_observer);
 }
 
-void TaskQueueManager::SetTimeSourceForTesting(
-    scoped_ptr<base::TickClock> time_source) {
-  DCHECK(main_thread_checker_.CalledOnValidThread());
-  time_source_ = time_source.Pass();
-}
-
 bool TaskQueueManager::GetAndClearSystemIsQuiescentBit() {
   bool task_was_run = task_was_run_on_quiescence_monitored_queue_;
   task_was_run_on_quiescence_monitored_queue_ = false;
   return !task_was_run;
 }
 
-base::TimeTicks TaskQueueManager::Now() const {
-  return time_source_->NowTicks();
+base::TickClock* TaskQueueManager::tick_clock() const {
+  return main_task_runner_.get();
 }
 
 int TaskQueueManager::GetNextSequenceNumber() {