GN files for running Closure Compiler

chrome/browser/resources/chromeos/braille_ime/BUILD.gn

 # 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_chromeos)
+import("//third_party/closure_compiler/compile_js2.gni")
 
 copy("braille_ime_manifest") {
+  assert(is_chromeos)
+
   sources = [
     "manifest.json",
   ]
   outputs = [
     "$root_out_dir/resources/chromeos/braille_ime/manifest.json",
   ]
 }
+
+compile_js("compile_externs") {
+  sources = [
+    "externs.js",
+  ]
+}