Implement qpSum stat for video send ssrc stats.

Implemented as defined by this pull request: https://github.com/w3c/webrtc-stats/pull/70

BUG=webrtc:6541

Review-Url: https://codereview.webrtc.org/2430603003
Cr-Commit-Position: refs/heads/master@{#14851}
diff --git a/webrtc/api/statscollector_unittest.cc b/webrtc/api/statscollector_unittest.cc
index 8c82811..10fe1e3 100644
--- a/webrtc/api/statscollector_unittest.cc
+++ b/webrtc/api/statscollector_unittest.cc
@@ -1926,6 +1926,7 @@
   // Construct a stats value to read.
   video_sender_info.add_ssrc(1234);
   video_sender_info.frames_encoded = 10;
+  video_sender_info.qp_sum = rtc::Optional<uint64_t>(11);
   stats_read.senders.push_back(video_sender_info);
 
   EXPECT_CALL(session_, video_channel()).WillRepeatedly(Return(&video_channel));
@@ -1937,6 +1938,8 @@
   EXPECT_EQ(rtc::ToString(video_sender_info.frames_encoded),
             ExtractSsrcStatsValue(reports,
                                   StatsReport::kStatsValueNameFramesEncoded));
+  EXPECT_EQ(rtc::ToString(*video_sender_info.qp_sum),
+            ExtractSsrcStatsValue(reports, StatsReport::kStatsValueNameQpSum));
 }
 
 // This test verifies that stats are correctly set in video receive ssrc stats.