Introduce media::TimeSource.

media/base/wall_clock_time_source.h

+// Copyright 2014 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 MEDIA_BASE_WALL_CLOCK_TIME_SOURCE_H_
+#define MEDIA_BASE_WALL_CLOCK_TIME_SOURCE_H_
+
+#include "base/memory/scoped_ptr.h"
+#include "media/base/media_export.h"
+#include "media/base/time_source.h"
+
+namespace base {
+class TickClock;
+}
+
+namespace media {
+
+// A time source that uses interpolation based on the system clock.
+class MEDIA_EXPORT WallClockTimeSource : public TimeSource {
+ public:
+  WallClockTimeSource();
+  virtual ~WallClockTimeSource();
+
+  // TimeSource implementation.
+  virtual void StartTicking() OVERRIDE;
+  virtual void StopTicking() OVERRIDE;
+  virtual void SetPlaybackRate(float playback_rate) OVERRIDE;
+  virtual void SetMediaTime(base::TimeDelta time) OVERRIDE;
+  virtual base::TimeDelta CurrentMediaTime() OVERRIDE;
+
+  void SetTickClockForTesting(scoped_ptr<base::TickClock> tick_clock);
+
+ private:
+  scoped_ptr<base::TickClock> tick_clock_;
+  bool ticking_;
+
+  // While ticking we can interpolate the current media time by measuring the
+  // delta between our reference ticks and the current system ticks and scaling
+  // that time by the playback rate.
+  float playback_rate_;
+  base::TimeDelta base_time_;
+  base::TimeTicks reference_wall_ticks_;
+
+  DISALLOW_COPY_AND_ASSIGN(WallClockTimeSource);
+};
+
+}  // namespace media
+
+#endif  // MEDIA_BASE_WALL_CLOCK_TIME_SOURCE_H_