Implement TLS client auth in the OS X OpenSSL port.

net/BUILD.gn

Index: net/BUILD.gn
diff --git a/net/BUILD.gn b/net/BUILD.gn
index 9d36de8429ea22c1cd5080aa80441da4bac04e45..ac13764b34f0cf74d0b6c4da5a6f933009c329a1 100644
--- a/net/BUILD.gn
+++ b/net/BUILD.gn
@@ -270,15 +270,26 @@ component("net") {
       "quic/crypto/p256_key_exchange_openssl.cc",
       "quic/crypto/scoped_evp_aead_ctx.cc",
       "quic/crypto/scoped_evp_aead_ctx.h",
-      "socket/openssl_ssl_util.cc",
-      "socket/openssl_ssl_util.h",
       "socket/ssl_client_socket_openssl.cc",
       "socket/ssl_client_socket_openssl.h",
       "socket/ssl_server_socket_openssl.cc",
       "socket/ssl_server_socket_openssl.h",
       "socket/ssl_session_cache_openssl.cc",
       "socket/ssl_session_cache_openssl.h",
+      "ssl/openssl_platform_key.h",
+      "ssl/openssl_ssl_util.cc",
+      "ssl/openssl_ssl_util.h",
     ]
+    if (is_mac) {
+      sources -= [
+        "ssl/openssl_platform_key_mac.cc",
+      ]
+    }
+    if (is_win) {
+      sources -= [
+        "ssl/openssl_platform_key_win.cc",
+      ]
+    }
   }
 
   if (!use_openssl_certs) {