Provision to start base::SimpleTestTickClock at initial ticks

ash/shelf/shelf_button_pressed_metric_tracker_unittest.cc

Index: ash/shelf/shelf_button_pressed_metric_tracker_unittest.cc
diff --git a/ash/shelf/shelf_button_pressed_metric_tracker_unittest.cc b/ash/shelf/shelf_button_pressed_metric_tracker_unittest.cc
index 010161805ad0ad2c5a6e68108e3b836e5ae6d353..1a21c56b340d83fd7c006c554b9cb08464be897a 100644
--- a/ash/shelf/shelf_button_pressed_metric_tracker_unittest.cc
+++ b/ash/shelf/shelf_button_pressed_metric_tracker_unittest.cc
@@ -118,13 +118,12 @@ void ShelfButtonPressedMetricTrackerTest::SetUp() {
 
   ShelfButtonPressedMetricTrackerTestAPI test_api(metric_tracker_);
 
-  scoped_ptr<base::TickClock> test_tick_clock(new base::SimpleTestTickClock());
-  tick_clock_ = static_cast<base::SimpleTestTickClock*>(test_tick_clock.get());
-  test_api.SetTickClock(std::move(test_tick_clock));
-
   // Ensure the TickClock->NowTicks() doesn't return base::TimeTicks because
   // ShelfButtonPressedMetricTracker interprets that value as unset.
-  tick_clock_->Advance(base::TimeDelta::FromMilliseconds(100));
+  scoped_ptr<base::TickClock> test_tick_clock(new base::SimpleTestTickClock(
+      base::TimeTicks() + base::TimeDelta::FromMilliseconds(100)));
+  tick_clock_ = static_cast<base::SimpleTestTickClock*>(test_tick_clock.get());
+  test_api.SetTickClock(std::move(test_tick_clock));
 }
 
 void ShelfButtonPressedMetricTrackerTest::TearDown() {