Adding macro to enable changing SSL library (Part 1)

build/config/crypto.gni

 # 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.
 
 # This file declares build flags for the SSL library configuration.
 #
 # TODO(brettw) this should probably be moved to src/crypto or somewhere, and
 # the global build dependency on it should be removed.
 #
 # PLEASE TRY TO AVOID ADDING FLAGS TO THIS FILE in cases where grit isn't
 # required. See the declare_args block of BUILDCONFIG.gn for advice on how
 # to set up feature flags.
 
 declare_args() {
   # Use OpenSSL instead of NSS. This is used for all platforms but iOS. (See
   # http://crbug.com/338886).
   use_openssl = !is_ios
 }
 
 # True when we're using OpenSSL for representing certificates. When targeting
 # Android, the platform certificate library is used for certificate
 # verification. On NaCl, verification isn't supported. On other targets, this
 # flag also enables OpenSSL for certificate verification, but this configuration
 # is unsupported.
 use_openssl_certs = is_android || is_nacl
 
-# True if NSS is used for certificate verification. Note that this is
-# independent from use_openssl. It is possible to use OpenSSL for the crypto
-# library, but NSS for the platform certificate library.
+# True if NSS is used for certificate handling. Note that this is independent
+# from use_openssl. It is possible to use OpenSSL for the crypto library, but
+# NSS for the platform certificate library.
 use_nss_certs = is_linux
+
+# True if NSS is used for certificate verification. On non-OpenSSL iOS
+# configurations, certificates use the operating system library, but the
+# verifier uses the bundled copy of NSS.
+use_nss_verifier = use_nss_certs || (is_ios && !use_openssl)