Add script files of the separated Gallery.app.

chrome/browser/resources/gallery/js/util.js

Index: chrome/browser/resources/gallery/js/util.js
diff --git a/chrome/browser/resources/gallery/js/util.js b/chrome/browser/resources/gallery/js/util.js
new file mode 100644
index 0000000000000000000000000000000000000000..21151e325cdddbb4b777fb5cbc6d09e7e59f7c39
--- /dev/null
+++ b/chrome/browser/resources/gallery/js/util.js
@@ -0,0 +1,15 @@
+// Copyright (c) 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.
+
+'use strict';
+
+/**
+ * Shows an error message if the promise is rejected.
+ * @param {Promise} promise Promise
+ */
+function trackPromise(promise) {
+  promise.catch(function(error) {
+    console.error(error.stack || error);
+  });
+}