Only send sigalgs extension in 1.2-capable ClientHellos.

BUG=https://code.google.com/p/webrtc/issues/detail?id=4223

Change-Id: I88eb036fdc6da17bc6a5179df02f35486abe9add
Reviewed-on: https://boringssl-review.googlesource.com/3030
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/ssl/test/runner/handshake_server.go b/ssl/test/runner/handshake_server.go
index b61292e..1234a57 100644
--- a/ssl/test/runner/handshake_server.go
+++ b/ssl/test/runner/handshake_server.go
@@ -172,6 +172,11 @@
 	}
 	c.clientVersion = hs.clientHello.vers
 
+	// Reject < 1.2 ClientHellos with signature_algorithms.
+	if c.clientVersion < VersionTLS12 && len(hs.clientHello.signatureAndHashes) > 0 {
+		return false, fmt.Errorf("tls: client included signature_algorithms before TLS 1.2")
+	}
+
 	c.vers, ok = config.mutualVersion(hs.clientHello.vers)
 	if !ok {
 		c.sendAlert(alertProtocolVersion)