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.cc b/webrtc/api/statscollector.cc
index 819f31b..a40835c 100644
--- a/webrtc/api/statscollector.cc
+++ b/webrtc/api/statscollector.cc
@@ -253,6 +253,8 @@
                      (info.adapt_reason & 0x1) > 0);
   report->AddBoolean(StatsReport::kStatsValueNameViewLimitedResolution,
                      (info.adapt_reason & 0x4) > 0);
+  if (info.qp_sum)
+    report->AddInt(StatsReport::kStatsValueNameQpSum, *info.qp_sum);
 
   const IntForAdd ints[] = {
     { StatsReport::kStatsValueNameAdaptationChanges, info.adapt_changes },