Implement qpSum stat for video send ssrc stats.

webrtc/video_send_stream.h

Index: webrtc/video_send_stream.h
diff --git a/webrtc/video_send_stream.h b/webrtc/video_send_stream.h
index c0bee4ed3c9d5cdabea5a28530a43df8d24818c0..9fa059e62eaef0f4c672848d71c5643e4d814788 100644
--- a/webrtc/video_send_stream.h
+++ b/webrtc/video_send_stream.h
@@ -57,6 +57,7 @@ class VideoSendStream {
     int avg_encode_time_ms = 0;
     int encode_usage_percent = 0;
     uint32_t frames_encoded = 0;
+    uint32_t qp_sum = 0;

[hta-webrtc, 2016/10/21 09:13:12]

Should this initialize to -1, since we don't know at this time whether it will
be possible to generate qp_sum or not?

[sakal, 2016/10/21 14:17:26]

Made rtc::Optional.

     // Bitrate the encoder is currently configured to use due to bandwidth
     // limitations.
     int target_media_bitrate_bps = 0;