Add SSL support to the GN build

tools/gn/secondary/crypto/ssl/BUILD.gn

-# Copyright (c) 2013 The Chromium Authors. All rights reserved.
-# Use of this source code is governed by a BSD-style license that can be
-# found in the LICENSE file.
-
-import("//build/config/crypto.gni")
-import("//build/config/linux/pkg_config.gni")
-
-config("ssl_preprocessor_flags") {
-  if (use_openssl) {
-    defines = [ "USE_OPENSSL" ]
-  } else if (use_nss) {
-    defines = [ "USE_NSS" ]
-  }
-}
-
-# This meta-target will include the SSL library according to the build flags.
-group("metassl") {
-  direct_dependent_configs = [ ":ssl_preprocessor_flags" ]
-
-  if (use_openssl) {
-    assert(is_linux)
-    deps = [ "//third_party/openssl" ]
-  } else {
-    deps = [ "//net/third_party/nss:ssl" ]
-  }
-
-  forward_dependent_configs_from = deps
-}