Fix Bug: Video with Variable Frame Rate plays at incorrect speed.

media/base/moving_average.cc

Index: media/base/moving_average.cc
diff --git a/media/base/moving_average.cc b/media/base/moving_average.cc
index b78f4c0e3dc90a13b00068e31c031a4e39739813..5dd140377b953e76e0d7a2b86c024ab1dad32676 100644
--- a/media/base/moving_average.cc
+++ b/media/base/moving_average.cc
@@ -9,8 +9,7 @@
 namespace media {
 
 MovingAverage::MovingAverage(size_t depth)
-    : depth_(depth), count_(0), samples_(depth_) {
-}
+    : depth_(depth), count_(0), samples_(depth_), square_sum_us_(0) {}
 
 MovingAverage::~MovingAverage() {
 }
@@ -20,6 +19,8 @@ void MovingAverage::AddSample(base::TimeDelta sample) {
   // exceeds |depth_|.
   base::TimeDelta& oldest = samples_[count_++ % depth_];
   total_ += sample - oldest;
+  square_sum_us_ += sample.InMicroseconds() * sample.InMicroseconds() -
+                    oldest.InMicroseconds() * oldest.InMicroseconds();
   oldest = sample;
 }
 
@@ -32,9 +33,22 @@ base::TimeDelta MovingAverage::Average() const {
   return total_ / std::min(static_cast<uint64_t>(depth_), count_);
 }
 
+base::TimeDelta MovingAverage::Deviation() const {
+  DCHECK_GT(count_, 0u);
+
+  const double size = std::min(static_cast<uint64_t>(depth_), count_);
+  const double average_us = total_.InMicroseconds() / size;
+  double sqr_deviation_us = square_sum_us_ / size - average_us * average_us;
+  if (sqr_deviation_us < 0)
+    sqr_deviation_us = 0;
+
+  return base::TimeDelta::FromMicroseconds(sqrt(sqr_deviation_us));
+}
+
 void MovingAverage::Reset() {
   count_ = 0;
   total_ = base::TimeDelta();
+  square_sum_us_ = 0;
   std::fill(samples_.begin(), samples_.end(), base::TimeDelta());
 }