Add externs for gallery.

ui/file_manager/externs/platform.js

Index: ui/file_manager/externs/platform.js
diff --git a/ui/file_manager/externs/platform.js b/ui/file_manager/externs/platform.js
new file mode 100644
index 0000000000000000000000000000000000000000..476e3d17eb0538674d7a18ffc7cb0cf86de02659
--- /dev/null
+++ b/ui/file_manager/externs/platform.js
@@ -0,0 +1,13 @@
+// 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.
+
+// Externs which are common for all chrome packaged apps.
+
+/**
+ * @param {string} url
+ * @param {function(!Entry)} successCallback
+ * @param {function(!FileError)=} opt_errorCallback
+ */
+Window.prototype.webkitResolveLocalFileSystemURL =
+    function(url, successCallback, opt_errorCallback) {};