credentials: add TLS 1.3 cipher suites (#2596)

This lets the tests pass with Go1.12beta2.

credentials/tls13.go

@@ -0,0 +1,30 @@
+// +build go1.12
+
+/*
+ *
+ * Copyright 2019 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+package credentials
+
+import "crypto/tls"
+
+// This init function adds cipher suite constants only defined in Go 1.12.
+func init() {
+	cipherSuiteLookup[tls.TLS_AES_128_GCM_SHA256] = "TLS_AES_128_GCM_SHA256"
+	cipherSuiteLookup[tls.TLS_AES_256_GCM_SHA384] = "TLS_AES_256_GCM_SHA384"
+	cipherSuiteLookup[tls.TLS_CHACHA20_POLY1305_SHA256] = "TLS_CHACHA20_POLY1305_SHA256"
+}

test/channelz_test.go

@@ -1279,6 +1279,9 @@ var cipherSuites = []string{
 	"TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256",
 	"TLS_ECDHE_RSA_WITH_CHACHA20_POLY1305",
 	"TLS_ECDHE_ECDSA_WITH_CHACHA20_POLY1305",
+	"TLS_AES_128_GCM_SHA256",
+	"TLS_AES_256_GCM_SHA384",
+	"TLS_CHACHA20_POLY1305_SHA256",
 }
 
 func (s) TestCZSocketGetSecurityValueTLS(t *testing.T) {