Add type annotations to gallery/js/gallery.js.

ui/file_manager/externs/gallery_foreground.js

Index: ui/file_manager/externs/gallery_foreground.js
diff --git a/ui/file_manager/externs/gallery_foreground.js b/ui/file_manager/externs/gallery_foreground.js
new file mode 100644
index 0000000000000000000000000000000000000000..b308725bae934e61377c512a6ab041de7280732d
--- /dev/null
+++ b/ui/file_manager/externs/gallery_foreground.js
@@ -0,0 +1,25 @@
+// 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.
+
+/**
+ * @typedef {function(number, number, boolean)}
+ */
+ImageBuffer.DragHandler;
+
+/**
+ * BackgroundComponents.
+ * @constructor
+ * @struct
+ */
+function BackgroundComponents() {
+  /**
+   * @type {!Object.<string, string>}
+   */
+  this.stringData;
+
+  /**
+   * @type {!VolumeManager}
+   */
+  this.volumeManager;
+}