Change base::TickClock to a ref counted class.

base/time/tick_clock.h

Index: base/time/tick_clock.h
diff --git a/base/time/tick_clock.h b/base/time/tick_clock.h
index 3e53d832857eefb46d708892ce3cb7cc170221a4..f0510b0cc5a0603420666a51568c2c0023c4265c 100644
--- a/base/time/tick_clock.h
+++ b/base/time/tick_clock.h
@@ -6,6 +6,7 @@
 #define BASE_TICK_CLOCK_H_
 
 #include "base/base_export.h"
+#include "base/memory/ref_counted.h"
 #include "base/time/time.h"
 
 namespace base {
@@ -24,15 +25,17 @@ namespace base {
 // simple test implementation.
 //
 // See Clock (base/time/clock.h) for the equivalent interface for Times.
-class BASE_EXPORT TickClock {
+class BASE_EXPORT TickClock : public RefCounted<TickClock> {
  public:
-  virtual ~TickClock();
-
   // NowTicks() must be safe to call from any thread.  The caller may
   // assume that NowTicks() is monotonic (but not strictly monotonic).
   // In other words, the returned TimeTicks will never decrease with
   // time, although they might "stand still".
   virtual TimeTicks NowTicks() = 0;
+
+ protected:
+  friend class base::RefCounted<TickClock>;
+  virtual ~TickClock();
 };
 
 }  // namespace base