Added A/V sync tests with drifting clocks.

webrtc/test/drifting_clock.h

+/*
+ *  Copyright (c) 2016 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+#ifndef WEBRTC_TEST_DRIFTING_CLOCK_H_
+#define WEBRTC_TEST_DRIFTING_CLOCK_H_
+
+#include "webrtc/system_wrappers/include/clock.h"
+
+namespace webrtc {
+namespace test {
+class DriftingClock : public Clock {
+ public:
+  // drift > 0 means DriftingClock goes slower than clock.

[pbos-webrtc, 2016/02/09 20:42:25]

Do drift_factor here and have 0 be still, 0.5 be half-speed and 2.0 be twice as
fast. I think that's easier as an interface from the outside. Feel free to
implement it the same internally but calculate drift_ if that makes things
easier though.

[danilchap, 2016/02/10 12:40:35]

Done.
Is 'speed' instead of 'drift_factor' a clear enough word?

+  // drift = 1.0 means DriftingClock goes twice as slow as clock.
+  // drift < 0 means DriftingClock goes faster than clock.
+  // drift = -0.5 means DriftingClock goes twice as fast as clock.
+  DriftingClock(Clock* clock, float drift);
+
+  int64_t TimeInMilliseconds() const override;
+  int64_t TimeInMicroseconds() const override;
+  void CurrentNtp(uint32_t& seconds, uint32_t& fractions) const override;
+  int64_t CurrentNtpInMilliseconds() const override;
+
+ private:
+  float Drift() const;
+
+  Clock* const clock_;
+  const float drift_;
+  int64_t start_time_;
+};
+}  // namespace test
+}  // namespace webrtc
+
+#endif  // WEBRTC_TEST_DRIFTING_CLOCK_H_