Support a user-provided string for the TLS ALPN extension.

webrtc/pc/iceserverparsing.cc

Index: webrtc/pc/iceserverparsing.cc
diff --git a/webrtc/pc/iceserverparsing.cc b/webrtc/pc/iceserverparsing.cc
index d9f4885c2b87c29ea440bff4c0dbc06c43ff7efd..00e9f2590f5909c317b8776a01a14f6094aa3344 100644
--- a/webrtc/pc/iceserverparsing.cc
+++ b/webrtc/pc/iceserverparsing.cc
@@ -257,6 +257,8 @@ static RTCErrorType ParseIceServerUrl(
         config.tls_cert_policy =
             cricket::TlsCertPolicy::TLS_CERT_POLICY_INSECURE_NO_CHECK;
       }
+      config.tls_alpn_protocols = server.tls_alpn_protocols;
+
       turn_servers->push_back(config);
       break;
     }