Mojo: Add a GN target for the shell interface.

mojo/public/interfaces/shell/BUILD.gn

Index: mojo/public/interfaces/shell/BUILD.gn
diff --git a/mojo/public/interfaces/shell/BUILD.gn b/mojo/public/interfaces/shell/BUILD.gn
new file mode 100644
index 0000000000000000000000000000000000000000..4308439d25745e12b4d334f99885276627245f58
--- /dev/null
+++ b/mojo/public/interfaces/shell/BUILD.gn
@@ -0,0 +1,11 @@
+# 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.
+
+import("//mojo/public/tools/bindings/mojom.gni")
+
+mojom("shell_bindings") {
+  sources = [
+    "shell.mojom",
+  ]
+}