Set wallpaper from right click menu in Files app.

ui/file_manager/externs/chrome_wallpaper.js

Index: ui/file_manager/externs/chrome_wallpaper.js
diff --git a/ui/file_manager/externs/chrome_wallpaper.js b/ui/file_manager/externs/chrome_wallpaper.js
new file mode 100644
index 0000000000000000000000000000000000000000..f5ffa4ff6feaf93e4da5491438f2962d35e2dba4
--- /dev/null
+++ b/ui/file_manager/externs/chrome_wallpaper.js
@@ -0,0 +1,28 @@
+// 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 = {};
+
+/**
+ * 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) {};