RTCCertificateStats[1] added.

[1] https://w3c.github.io/webrtc-stats/#certificatestats-dict*

BUG=chromium:627816, chromium:629436
NOTRY=True

Review-Url: https://codereview.webrtc.org/2243123002
Cr-Commit-Position: refs/heads/master@{#14484}
diff --git a/webrtc/api/stats/rtcstats_objects.h b/webrtc/api/stats/rtcstats_objects.h
index ac480a9..6ff9f99 100644
--- a/webrtc/api/stats/rtcstats_objects.h
+++ b/webrtc/api/stats/rtcstats_objects.h
@@ -17,6 +17,26 @@
 
 namespace webrtc {
 
+// https://w3c.github.io/webrtc-stats/#certificatestats-dict*
+class RTCCertificateStats : public RTCStats {
+ public:
+  RTCCertificateStats(const std::string& id, int64_t timestamp_us);
+  RTCCertificateStats(std::string&& id, int64_t timestamp_us);
+
+  WEBRTC_RTCSTATS_IMPL(RTCStats, RTCCertificateStats,
+      &fingerprint,
+      &fingerprint_algorithm,
+      &base64_certificate,
+      &issuer_certificate_id);
+
+  RTCStatsMember<std::string> fingerprint;
+  RTCStatsMember<std::string> fingerprint_algorithm;
+  RTCStatsMember<std::string> base64_certificate;
+  RTCStatsMember<std::string> issuer_certificate_id;
+};
+
+// https://w3c.github.io/webrtc-stats/#pcstats-dict*
+// TODO(hbos): Tracking bug crbug.com/636818
 class RTCPeerConnectionStats : public RTCStats {
  public:
   RTCPeerConnectionStats(const std::string& id, int64_t timestamp_us);