Add externs for gallery.

ui/file_manager/externs/gallery_background.js

Index: ui/file_manager/externs/gallery_background.js
diff --git a/ui/file_manager/externs/gallery_background.js b/ui/file_manager/externs/gallery_background.js
index 893d16172069718ad002c105e146b239300f4778..6e9ae8495e617a0ef441ba4eb6f53b8a9364d72b 100644
--- a/ui/file_manager/externs/gallery_background.js
+++ b/ui/file_manager/externs/gallery_background.js
@@ -20,3 +20,10 @@ window.initialize = function(backgroundComponents) {};
  * @param {!Array.<!Entry>} selectedEntries Array of selected entries.
  */
 window.loadEntries = function(entries, selectedEntries) {};
+
+/**
+ * This definition is required by
+ * ui/file_manager/file_manager/common/js/util.js.
+ * @type {string}
+ */
+Window.prototype.appID;