Improve audio/video sync during underflow, reduce underflow frequency.

media/base/wall_clock_time_source.cc

Index: media/base/wall_clock_time_source.cc
diff --git a/media/base/wall_clock_time_source.cc b/media/base/wall_clock_time_source.cc
index 1c59be948bbf6f22b2fe8472d93d3ac78f29e5c5..33ec372e5008ea732c1f7a600c15fbdc8d832841 100644
--- a/media/base/wall_clock_time_source.cc
+++ b/media/base/wall_clock_time_source.cc
@@ -20,16 +20,16 @@ void WallClockTimeSource::StartTicking() {
   base::AutoLock auto_lock(lock_);
   DCHECK(!ticking_);
   ticking_ = true;
-  reference_wall_ticks_ = tick_clock_->NowTicks();
+  reference_time_ = tick_clock_->NowTicks();
 }
 
 void WallClockTimeSource::StopTicking() {
   DVLOG(1) << __FUNCTION__;
   base::AutoLock auto_lock(lock_);
   DCHECK(ticking_);
-  base_time_ = CurrentMediaTime_Locked();
+  base_timestamp_ = CurrentMediaTime_Locked();
   ticking_ = false;
-  reference_wall_ticks_ = tick_clock_->NowTicks();
+  reference_time_ = tick_clock_->NowTicks();
 }
 
 void WallClockTimeSource::SetPlaybackRate(double playback_rate) {
@@ -38,8 +38,8 @@ void WallClockTimeSource::SetPlaybackRate(double playback_rate) {
   // Estimate current media time using old rate to use as a new base time for
   // the new rate.
   if (ticking_) {
-    base_time_ = CurrentMediaTime_Locked();
-    reference_wall_ticks_ = tick_clock_->NowTicks();
+    base_timestamp_ = CurrentMediaTime_Locked();
+    reference_time_ = tick_clock_->NowTicks();
   }
 
   playback_rate_ = playback_rate;
@@ -49,7 +49,7 @@ void WallClockTimeSource::SetMediaTime(base::TimeDelta time) {
   DVLOG(1) << __FUNCTION__ << "(" << time.InMicroseconds() << ")";
   base::AutoLock auto_lock(lock_);
   CHECK(!ticking_);
-  base_time_ = time;
+  base_timestamp_ = time;
 }
 
 base::TimeDelta WallClockTimeSource::CurrentMediaTime() {
@@ -61,29 +61,34 @@ bool WallClockTimeSource::GetWallClockTimes(
     const std::vector<base::TimeDelta>& media_timestamps,
     std::vector<base::TimeTicks>* wall_clock_times) {
   base::AutoLock auto_lock(lock_);
-  if (!ticking_ || !playback_rate_)
-    return false;
-
   DCHECK(wall_clock_times->empty());
-  wall_clock_times->reserve(media_timestamps.size());
-  for (const auto& media_timestamp : media_timestamps) {
-    wall_clock_times->push_back(
-        reference_wall_ticks_ +
-        base::TimeDelta::FromMicroseconds(
-            (media_timestamp - base_time_).InMicroseconds() / playback_rate_));
+
+  if (media_timestamps.empty()) {
+    wall_clock_times->push_back(reference_time_);
+  } else {
+    // When playback is paused (rate is zero), assume a rate of 1.0.
+    const double playback_rate = playback_rate_ ? playback_rate_ : 1.0;
+
+    wall_clock_times->reserve(media_timestamps.size());
+    for (const auto& media_timestamp : media_timestamps) {
+      wall_clock_times->push_back(reference_time_ +
+                                  (media_timestamp - base_timestamp_) /
+                                      playback_rate);
+    }
   }
-  return true;
+
+  return playback_rate_ && ticking_;
 }
 
 base::TimeDelta WallClockTimeSource::CurrentMediaTime_Locked() {
   lock_.AssertAcquired();
   if (!ticking_ || !playback_rate_)
-    return base_time_;
+    return base_timestamp_;
 
   base::TimeTicks now = tick_clock_->NowTicks();
-  return base_time_ +
+  return base_timestamp_ +
          base::TimeDelta::FromMicroseconds(
-             (now - reference_wall_ticks_).InMicroseconds() * playback_rate_);
+             (now - reference_time_).InMicroseconds() * playback_rate_);
 }
 
 }  // namespace media