Change base::TickClock to a ref counted class.

base/test/simple_test_tick_clock.h

Index: base/test/simple_test_tick_clock.h
diff --git a/base/test/simple_test_tick_clock.h b/base/test/simple_test_tick_clock.h
index c67eb558ff781a86be721728d5e1aeca25cf4bcd..56e6a22b04674b31a77e653009ce9db5b858a902 100644
--- a/base/test/simple_test_tick_clock.h
+++ b/base/test/simple_test_tick_clock.h
@@ -19,7 +19,6 @@ class SimpleTestTickClock : public TickClock {
  public:
   // Starts off with a clock set to TimeTicks().
   SimpleTestTickClock();
-  virtual ~SimpleTestTickClock();
 
   virtual TimeTicks NowTicks() override;
 
@@ -27,6 +26,8 @@ class SimpleTestTickClock : public TickClock {
   void Advance(TimeDelta delta);
 
  private:
+  virtual ~SimpleTestTickClock();
+
   // Protects |now_ticks_|.
   Lock lock_;