stats: add dtlsRole to transport

https://w3c.github.io/webrtc-stats/#dom-rtctransportstats-dtlsrole

BUG=webrtc:13978

Change-Id: Ib158427d2df0307884381bdd46c411f60f56a371
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/259761
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Philipp Hancke <philipp.hancke@googlemail.com>
Cr-Commit-Position: refs/heads/main@{#36730}
diff --git a/api/stats/rtcstats_objects.h b/api/stats/rtcstats_objects.h
index bcf606f..a6d78f8 100644
--- a/api/stats/rtcstats_objects.h
+++ b/api/stats/rtcstats_objects.h
@@ -90,6 +90,13 @@
   static const char* const kScreenshare;
 };
 
+// https://w3c.github.io/webrtc-stats/#dom-rtcdtlsrole
+struct RTCDtlsRole {
+  static const char* const kUnknown;
+  static const char* const kClient;
+  static const char* const kServer;
+};
+
 // https://w3c.github.io/webrtc-stats/#certificatestats-dict*
 class RTC_EXPORT RTCCertificateStats final : public RTCStats {
  public:
@@ -689,6 +696,7 @@
   RTCStatsMember<std::string> remote_certificate_id;
   RTCStatsMember<std::string> tls_version;
   RTCStatsMember<std::string> dtls_cipher;
+  RTCStatsMember<std::string> dtls_role;
   RTCStatsMember<std::string> srtp_cipher;
   RTCStatsMember<uint32_t> selected_candidate_pair_changes;
 };