Added A/V sync tests with drifting clocks.

webrtc/test/drifting_clock.cc

+/*
+ *  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.
+ */
+
+#include "webrtc/test/drifting_clock.h"
+#include "webrtc/base/checks.h"
+
+namespace webrtc {
+namespace test {
+
+DriftingClock::DriftingClock(Clock* clock, float drift)
+    : clock_(clock), drift_(drift), start_time_(-1) {

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

make start_time_ const and set in init list

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

Done.

+  RTC_CHECK(clock);
+  RTC_CHECK_GT(drift, -1.0f);
+  start_time_ = clock_->TimeInMicroseconds();
+}
+
+float DriftingClock::Drift() const {
+  int64_t now = clock_->TimeInMicroseconds();
+  RTC_DCHECK_GE(now, start_time_);
+  return (now - start_time_) * drift_;
+}
+
+int64_t DriftingClock::TimeInMilliseconds() const {
+  return clock_->TimeInMilliseconds() + Drift() / 1000.;
+}
+
+int64_t DriftingClock::TimeInMicroseconds() const {
+  return clock_->TimeInMicroseconds() + Drift();
+}
+
+void DriftingClock::CurrentNtp(uint32_t& seconds, uint32_t& fractions) const {
+  const double kNtpFracPerMicroSecond = 4294.967296;  // = 2^32 / 10^6

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

Can you link to something or explain that NTP are fractions of 2^32 or something
in the comment as well?

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

Added small comment stating NTP is based on 1/2^32 seconds. Without explanation
why.

+
+  clock_->CurrentNtp(seconds, fractions);
+  uint64_t total_fractions = (static_cast<uint64_t>(seconds) << 32) | fractions;
+  total_fractions += Drift() * kNtpFracPerMicroSecond;
+  seconds = total_fractions >> 32;
+  fractions = static_cast<uint32_t>(total_fractions);
+}
+
+int64_t DriftingClock::CurrentNtpInMilliseconds() const {
+  return clock_->CurrentNtpInMilliseconds() + Drift() / 1000.;
+}
+}  // namespace test
+}  // namespace webrtc