Add a Clock and TickClock interface for mocking out time

base/default_tick_clock.h

Index: base/default_tick_clock.h
diff --git a/base/default_tick_clock.h b/base/default_tick_clock.h
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index 0000000000000000000000000000000000000000..76fae6d58f452f4f97e56794fed484d2c68eaa73
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+++ b/base/default_tick_clock.h
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+// 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/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;
+
+  // Returns a single lazy-initialized instance of DefaultTickClock.
+  // But you can create your own DefaultTickClock object if you wish.
+  static DefaultTickClock* Get();
+};
+
+}  // namespace base
+
+#endif  // BASE_DEFAULT_CLOCK_H_