Land prep work to enable NaCl in the Linux x64 GN builds.

build/config/features.gni

 # Copyright 2014 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 contains UI-related build flags. It should theoretically be in the
 # src/ui directory and only things that depend on the ui module should get the
 # definitions.
 #
 # However, today we have many "bad" dependencies on some of these flags from,
 # e.g. base, so they need to be global.
 #
 # See also build/config/ui.gni
 
 if (is_android) {
   import("//build/config/android/config.gni")
 }
 
 declare_args() {
   # Multicast DNS.
   enable_mdns = is_win || is_linux
 
   enable_plugins = !is_android && !is_ios
 
   # Enables Native Client support.
-  # TODO(GYP) enable this when nacl works in GN.
+  # TODO(GYP): Get NaCl linking on other platforms.
+  # Also, see if we can always get rid of enable_nacl_untrusted and
+  # enable_pnacl and always build them if enable_nacl is true.
+  # The "is_nacl" part of the condition is needed to ensure that
+  # the untrusted code is built properly; arguably it should be
+  # guarded by "is_nacl" directly rather than enable_nacl_untrusted, but
+  # this will go away when Mac and Win are working and we can just use
+  # the commented out logic.
+  # enable_nacl = !is_ios && !is_android
+  # Currently this *should* work:
+  # enable_nacl = (is_linux && build_cpu_arch == "x64") || is_nacl
   enable_nacl = false
-  #enable_nacl = (!is_ios && !is_android)
+  enable_nacl_untrusted = enable_nacl
+  enable_pnacl = enable_nacl_untrusted
 
   # If debug_devtools is set to true, JavaScript files for DevTools are stored
   # as is and loaded from disk. Otherwise, a concatenated file is stored in
   # resources.pak. It is still possible to load JS files from disk by passing
   # --debug-devtools cmdline switch.
   debug_devtools = false
 
   # Enables WebRTC.
   # TODO(GYP) make mac and android work.
   enable_webrtc = !is_ios && !is_mac && !is_android
@@ skipping 121 matching lines @@
 
 # Option controlling the use of GConf (the classic GNOME configuration
 # system).
 # TODO(GYP) also require !embedded to enable.
 use_gconf = is_linux && !is_chromeos
 
 # Hangout services is an extension that adds extra features to Hangouts.
 # For official GYP builds, this flag is set, it will likely need to be
 # parameterized in the future for a similar use.
 enable_hangout_services_extension = false