Change base::TickClock to a ref counted class.

base/time/default_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_DEFAULT_TICK_CLOCK_H_
 #define BASE_DEFAULT_TICK_CLOCK_H_
 
 #include "base/base_export.h"
 #include "base/compiler_specific.h"
 #include "base/time/tick_clock.h"
 
 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
 
 #endif  // BASE_DEFAULT_CLOCK_H_