Add QP sum stats for received streams.

webrtc/video/receive_statistics_proxy.cc

Index: webrtc/video/receive_statistics_proxy.cc
diff --git a/webrtc/video/receive_statistics_proxy.cc b/webrtc/video/receive_statistics_proxy.cc
index 68050ba6a8a440dcc3eb31a4edf22127d99949c0..b78ba66f95c61cb61bba12b42591f1f910aefb4d 100644
--- a/webrtc/video/receive_statistics_proxy.cc
+++ b/webrtc/video/receive_statistics_proxy.cc
@@ -395,11 +395,20 @@ void ReceiveStatisticsProxy::DataCountersUpdated(
   }
 }
 
-void ReceiveStatisticsProxy::OnDecodedFrame() {
+void ReceiveStatisticsProxy::OnDecodedFrame(int qp) {
   uint64_t now = clock_->TimeInMilliseconds();
 
   rtc::CritScope lock(&crit_);
   ++stats_.frames_decoded;
+  if (qp != -1) {

[sprang_webrtc, 2017/01/25 15:25:41]

Maybe we should dcheck on the off chance qp < -1 ?
Is 0 a valid value btw?

[sakal, 2017/01/25 15:34:45]

Sure, I can add that once we decide whether to use a integer or rtc::Optional. 0
is a valid value for all codecs.

[sakal, 2017/01/26 09:33:31]

Done.

+    if (!stats_.qp_sum) {
+      RTC_DCHECK_EQ(stats_.frames_decoded, 1);
+      stats_.qp_sum = rtc::Optional<uint64_t>(0);
+    }
+    *stats_.qp_sum += qp;
+  } else {
+    RTC_DCHECK(!stats_.qp_sum);

[sprang_webrtc, 2017/01/25 15:25:41]

Any chance we could hit this if we change codec on the fly, such as falling back
from hw to sw encoder or vice versa?

[sakal, 2017/01/25 15:34:45]

Yes, I think that is possible if one supports providing QP but another doesn't.
This is of course non-ideal. Maybe in these cases we should just remove qp_sum
stat? Any comments on this?

[sprang_webrtc, 2017/01/25 15:59:14]

Yeah, logging a warning and resetting the stats sounds like the right thing to
do then.

[sakal, 2017/01/26 09:33:31]

Done.

+  }
   decode_fps_estimator_.Update(1, now);
   stats_.decode_frame_rate = decode_fps_estimator_.Rate(now).value_or(0);
 }