Add a Clock and TickClock interface for mocking out time

base/clock.h

Index: base/clock.h
diff --git a/base/clock.h b/base/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_CLOCK_H_
+#define BASE_CLOCK_H_
+
+#include "base/base_export.h"
+#include "base/time.h"
+
+namespace base {
+
+// A Clock is an interface for objects that vend Times.  It is
+// intended to be able to test the behavior of classes with respect to
+// time.
+//
+// See DefaultClock (base/default_clock.h) for the default
+// implementation that simply uses Time::Now().
+//
+// (An implementation that uses Time::SystemTime() should be added as
+// needed.)
+//
+// See SimpleTestClock (base/test/simple_test_clock.h) for a simple
+// test implementation.
+//
+// See TickClock (base/tick_clock.h) for the equivalent interface for
+// TimeTicks.
+class BASE_EXPORT Clock {
+ public:
+  virtual ~Clock();
+
+  // Now() must be safe to call from any thread.  The caller cannot
+  // make any ordering assumptions about the returned Time.  For
+  // example, the system clock may change to an earlier time.
+  virtual Time Now() = 0;
+};
+
+}  // namespace base
+
+#endif  // BASE_CLOCK_H_