Make gn check //mojo/public/* pass

mojo/public/cpp/system/BUILD.gn

Index: mojo/public/cpp/system/BUILD.gn
diff --git a/mojo/public/cpp/system/BUILD.gn b/mojo/public/cpp/system/BUILD.gn
new file mode 100644
index 0000000000000000000000000000000000000000..046ede0a3e0816e7367245c1cb7eddf0f70940b2
--- /dev/null
+++ b/mojo/public/cpp/system/BUILD.gn
@@ -0,0 +1,17 @@
+# 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.
+
+source_set("system") {
+  sources = [
+    "buffer.h",
+    "core.h",
+    "data_pipe.h",
+    "functions.h",
+    "handle.h",
+    "macros.h",
+    "message_pipe.h"
+  ]
+
+  public_deps = [ "//mojo/public/c/system" ]
+}