Expose OpenSSL's key_exchange_info in the content API

net/ssl/ssl_info.h

Index: net/ssl/ssl_info.h
diff --git a/net/ssl/ssl_info.h b/net/ssl/ssl_info.h
index da20817c2e688908c4f83e1541ecd5f3d44ba906..28ce82d0ae6cb1ddf7a20f6ab9fd47dfe6075903 100644
--- a/net/ssl/ssl_info.h
+++ b/net/ssl/ssl_info.h
@@ -60,6 +60,12 @@ class NET_EXPORT SSLInfo {
   // -1 means the security strength is unknown.
   int security_bits;
 
+  // Security information of the SSL connection handshake.
+  // The meaning depends on the cipher used, see BoringSSL's |SSL_SESSION|'s
+  // key_exchange_info for more information.
+  // A zero indicates that the value is unknown.
+  int key_exchange_info;
+
   // Information about the SSL connection itself. See
   // ssl_connection_status_flags.h for values. The protocol version,
   // ciphersuite, and compression in use are encoded within.