Add a Clock and TickClock interface for mocking out time

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
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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_TICK_CLOCK_H_
+#define BASE_TICK_CLOCK_H_
+
+#include "base/base_export.h"
+#include "base/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/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/clock.h) for the equivalent interface for Times.
+class BASE_EXPORT 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;
+};
+
+}  // namespace base
+
+#endif  // BASE_TICK_CLOCK_H_