Change base::TickClock to a ref counted class.

base/time/default_tick_clock.h

Index: base/time/default_tick_clock.h
diff --git a/base/time/default_tick_clock.h b/base/time/default_tick_clock.h
index b3d5a31dc40e91ededb2dd6fe3b96064cbbe546c..7d59bb4876e3509827ccc5e32425a1886902d970 100644
--- a/base/time/default_tick_clock.h
+++ b/base/time/default_tick_clock.h
@@ -14,10 +14,11 @@ namespace base {
 // DefaultClock is a Clock implementation that uses TimeTicks::Now().
 class BASE_EXPORT DefaultTickClock : public TickClock {
  public:
-  virtual ~DefaultTickClock();
-
   // Simply returns TimeTicks::Now().
   virtual TimeTicks NowTicks() override;
+
+ protected:
+  virtual ~DefaultTickClock();
 };
 
 }  // namespace base