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

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

Index: ppapi/native_client/src/untrusted/pnacl_irt_shim/BUILD.gn
diff --git a/ppapi/native_client/src/untrusted/pnacl_irt_shim/BUILD.gn b/ppapi/native_client/src/untrusted/pnacl_irt_shim/BUILD.gn
new file mode 100644
index 0000000000000000000000000000000000000000..4a6050d7309bbd49e5f6d8e08b5ff80971ecde81
--- /dev/null
+++ b/ppapi/native_client/src/untrusted/pnacl_irt_shim/BUILD.gn
@@ -0,0 +1,29 @@
+# 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("aot") {
+  sources = [
+    "irt_shim_ppapi.c",
+    "pnacl_shim.c",
+    "shim_entry.c",
+    "shim_ppapi.c",
+  ]
+}
+
+source_set("browser") {
+  sources = [
+    "shim_entry.c",
+    "shim_ppapi.c",
+  ]
+}
+
+source_set("irt") {
+  sources = [
+    "irt_shim_ppapi.c",
+    "pnacl_shim.c",
+  ]
+}