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

components/scheduler/renderer/task_cost_estimator.h

Index: components/scheduler/renderer/task_cost_estimator.h
diff --git a/components/scheduler/renderer/task_cost_estimator.h b/components/scheduler/renderer/task_cost_estimator.h
index 7fe68db67e4e734edee79520e21605df3ce565ad..fa0f4c4897eee39e24c9670a662fa39880fc7b09 100644
--- a/components/scheduler/renderer/task_cost_estimator.h
+++ b/components/scheduler/renderer/task_cost_estimator.h
@@ -20,7 +20,9 @@ namespace scheduler {
 class SCHEDULER_EXPORT TaskCostEstimator
     : public base::MessageLoop::TaskObserver {
  public:
-  TaskCostEstimator(int sample_count, double estimation_percentile);
+  TaskCostEstimator(base::TickClock* time_source,
+                    int sample_count,
+                    double estimation_percentile);
   ~TaskCostEstimator() override;
 
   base::TimeDelta expected_task_duration() const;
@@ -31,11 +33,9 @@ class SCHEDULER_EXPORT TaskCostEstimator
 
   void Clear();
 
-  void SetTimeSourceForTesting(scoped_ptr<base::TickClock> time_source);
-
  private:
   cc::RollingTimeDeltaHistory rolling_time_delta_history_;
-  scoped_ptr<base::TickClock> time_source_;
+  base::TickClock* time_source_;  // NOT OWNED
   int outstanding_task_count_;
   double estimation_percentile_;
   base::TimeTicks task_start_time_;