Add type signatures for enums in wallpaper API.

ui/file_manager/externs/chrome_wallpaper.js

 // Copyright 2016 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.
 /**
  * TODO(ryoh): REMOVE this file if we decide how to define annotations for this
  * API.
  * @see https://codereview.chromium.org/1679983003/
  */
  /**
- * @const
- * @see https://goo.gl/7dvJFW
- */
-chrome.wallpaper = {};
+  * @const

[fukino, 2016/02/29 05:51:43]

nit: every line has a leading space. please remove them.

[ryoh, 2016/02/29 05:54:52]

Done.

+  * @see https://goo.gl/7dvJFW
+  */
+ chrome.wallpaper = {};
 
-/**
- * Sets wallpaper to the image at url or wallpaperData with the specified
- * layout.
- * @param {{
- *    data: (ArrayBuffer|undefined),
- *    url: (string|undefined),
- *    layout: string,
- *    filename: string,
- *    thumbnail: (boolean|undefined)
- *  }} details
- * @param {function(ArrayBuffer=)} callback
- *
- */
- chrome.wallpaper.setWallpaper = function(details, callback) {};
+ /**
+  * @enum {string}
+  * @see https://goo.gl/7dvJFW#type-WallpaperLayout
+  */
+ chrome.wallpaper.WallpaperLayout = {
+   STRETCH: '',
+   CENTER: '',
+   CENTER_CROPPED: '',
+ };
+
+ /**
+  * Sets wallpaper to the image at url or wallpaperData with the specified
+  * layout.
+  * @param {{
+  *    data: (ArrayBuffer|undefined),
+  *    url: (string|undefined),
+  *    layout: (chrome.wallpaper.WallpaperLayout|string),
+  *    filename: string,
+  *    thumbnail: (boolean|undefined)
+  *  }} details
+  * @param {function(ArrayBuffer=)} callback
+  * @see https://goo.gl/7dvJFW#method-setWallpaper
+  */
+  chrome.wallpaper.setWallpaper = function(details, callback) {};