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

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

Index: ppapi/native_client/src/untrusted/pnacl_support_extension/BUILD.gn
diff --git a/ppapi/native_client/src/untrusted/pnacl_support_extension/BUILD.gn b/ppapi/native_client/src/untrusted/pnacl_support_extension/BUILD.gn
new file mode 100644
index 0000000000000000000000000000000000000000..50e9b7c5f2f4ca2f07acbf2ca819daa62e20be26
--- /dev/null
+++ b/ppapi/native_client/src/untrusted/pnacl_support_extension/BUILD.gn
@@ -0,0 +1,11 @@
+# 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.
+
+import("//build/config/features.gni")
+
+if (enable_nacl && enable_nacl_untrusted && enable_pnacl) {
+  group("pnacl_support_extension") {
+    # TODO(GYP): implement me ...
+  }
+}