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

ppapi/native_client/src/untrusted/irt_stub/BUILD.gn

Index: ppapi/native_client/src/untrusted/irt_stub/BUILD.gn
diff --git a/ppapi/native_client/src/untrusted/irt_stub/BUILD.gn b/ppapi/native_client/src/untrusted/irt_stub/BUILD.gn
new file mode 100644
index 0000000000000000000000000000000000000000..443f0802860f41ba1a185572b930d904cfbc7276
--- /dev/null
+++ b/ppapi/native_client/src/untrusted/irt_stub/BUILD.gn
@@ -0,0 +1,15 @@
+# Copyright 2015 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.
+
+assert(is_nacl,
+       "These targets must only be built using the untrusted NaCl toolchains.")
+
+source_set("ppapi_stub_lib") {
+  sources = [
+    "ppapi_plugin_main.c",
+    "ppapi_plugin_start.c",
+    "plugin_main_irt.c",
+    "thread_creator.c",
+  ]
+}