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

chrome/browser/nacl_host/test/BUILD.gn

Index: chrome/browser/nacl_host/test/BUILD.gn
diff --git a/chrome/browser/nacl_host/test/BUILD.gn b/chrome/browser/nacl_host/test/BUILD.gn
new file mode 100644
index 0000000000000000000000000000000000000000..fca5c76377681903d8f4a0741d4a3883fc13aa7a
--- /dev/null
+++ b/chrome/browser/nacl_host/test/BUILD.gn
@@ -0,0 +1,16 @@
+# 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 && (is_linux || is_win)) {
+  executable("mock_nacl_gdb") {
+    sources = [
+      "mock_nacl_gdb.cc",
+    ]
+    deps = [
+      "//base",
+    ]
+  }
+}