Change base::TickClock to a ref counted class.

base/time/tick_clock.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_TICK_CLOCK_H_
 #define BASE_TICK_CLOCK_H_
 
 #include "base/base_export.h"
+#include "base/memory/ref_counted.h"
 #include "base/time/time.h"
 
 namespace base {
 
 // A TickClock is an interface for objects that vend TimeTicks.  It is
 // intended to be able to test the behavior of classes with respect to
 // non-decreasing time.
 //
 // See DefaultTickClock (base/time/default_tick_clock.h) for the default
 // implementation that simply uses TimeTicks::Now().
 //
 // (Other implementations that use TimeTicks::HighResNow() or
 // TimeTicks::NowFromSystemTime() should be added as needed.)
 //
 // See SimpleTestTickClock (base/test/simple_test_tick_clock.h) for a
 // 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
 
 #endif  // BASE_TICK_CLOCK_H_