RTCRTPStreamStats.ssrc changed type to uint32_t.

As per PR: https://github.com/w3c/webrtc-stats/pull/157

BUG=webrtc:7065, webrtc:7066

Review-Url: https://codereview.webrtc.org/2675583003
Cr-Commit-Position: refs/heads/master@{#16471}
diff --git a/webrtc/api/stats/rtcstats_objects.h b/webrtc/api/stats/rtcstats_objects.h
index 6c24d67..29fcfaf 100644
--- a/webrtc/api/stats/rtcstats_objects.h
+++ b/webrtc/api/stats/rtcstats_objects.h
@@ -302,7 +302,7 @@
   RTCRTPStreamStats(const RTCRTPStreamStats& other);
   ~RTCRTPStreamStats() override;
 
-  RTCStatsMember<std::string> ssrc;
+  RTCStatsMember<uint32_t> ssrc;
   // TODO(hbos): When the remote case is supported |RTCStatsCollector| needs to
   // set this. crbug.com/657855, 657856
   RTCStatsMember<std::string> associate_stats_id;