Add an initial implementation of the downloads MD settings page.

chrome/browser/resources/settings/prefs/prefs_types.js

 // Copyright 2015 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.
 
 /**
  * @fileoverview Typedefs for CrSettingsPrefsElement.
  */
 
 var CrSettingsPrefs = {};
 
@@ skipping 15 matching lines @@
  *   screenMagnifier: boolean,
  *   autoclick: boolean,
  *   autoclickDelayMs: number,
  *   virtualKeyboard: boolean
  * }}
  */
 CrSettingsPrefs.A11y;
 
 /**
  * @typedef {{
+ *   downloadLocation: string,
+ *   promptForDownload: boolean
+ * }}
+ */
+CrSettingsPrefs.Downloads;
+
+/**
+ * @typedef {{
  *   enableTapDragging: boolean
  * }}
  */
 CrSettingsPrefs.Touchpad;