Enable X25519 by default in TLS.

BUG=571231

Change-Id: I73e39411ccdc817f172c7a94b7f70c448eed938f
Reviewed-on: https://boringssl-review.googlesource.com/6911
Reviewed-by: Adam Langley <agl@google.com>

ssl/t1_lib.c

diff --git a/ssl/t1_lib.c b/ssl/t1_lib.c
index d6ae87e..8cad57a 100644
--- a/ssl/t1_lib.c
+++ b/ssl/t1_lib.c
@@ -294,6 +294,7 @@
 }
 
 static const uint16_t eccurves_default[] = {
+    SSL_CURVE_ECDH_X25519,
     SSL_CURVE_SECP256R1,
     SSL_CURVE_SECP384R1,
 #if defined(BORINGSSL_ANDROID_SYSTEM)