Introduce media::TimeSource.

media/base/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_TIME_SOURCE_H_
+#define MEDIA_BASE_TIME_SOURCE_H_
+
+#include "base/time/time.h"
+#include "media/base/media_export.h"
+
+namespace media {
+
+// A TimeSource is capable of providing the current media time.
+class MEDIA_EXPORT TimeSource {
+ public:
+  TimeSource() {}
+  virtual ~TimeSource() {}
+
+  // Signal the time source to start ticking. It is expected that values from
+  // CurrentMediaTime() will start increasing.
+  virtual void StartTicking() = 0;
+
+  // Signal the time source to stop ticking. It is expected that values from
+  // CurrentMediaTime() will remain constant.
+  virtual void StopTicking() = 0;
+
+  // Updates the current playback rate. It is expected that values from
+  // CurrentMediaTime() will eventually reflect the new playback rate (e.g., the
+  // media time will advance at half speed if the rate was set to 0.5f).
+  virtual void SetPlaybackRate(float playback_rate) = 0;
+
+  // Sets the media time to start ticking from. Only valid to call while the
+  // time source is not ticking.
+  virtual void SetMediaTime(base::TimeDelta time) = 0;
+
+  // Returns the current media time.
+  virtual base::TimeDelta CurrentMediaTime() = 0;
+};
+
+}  // namespace media
+
+#endif  // MEDIA_BASE_TIME_SOURCE_H_