Add an initial implementation of the downloads MD settings page.

chrome/browser/resources/settings/prefs/prefs.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
  * 'cr-settings-prefs' is an element which serves as a model for
  * interaction with settings which are stored in Chrome's
  * Preferences.
  *
@@ skipping 16 matching lines @@
      */
     settings: null,
   },
 
   /** @override */
   created: function() {
     'use strict';
 
     this.settings = {};
     this.initializeA11y_();
+    this.initializeDownloads_();
     var observer = new ObjectObserver(this.settings);
     observer.open(this.propertyChangeCallback_.bind(this, 'settings'));
 
     // For all Object properties of settings, create an ObjectObserver.
     for (let key in this.settings) {
       if (typeof this.settings[key] == 'object') {
         let keyObserver = new ObjectObserver(this.settings[key]);
         keyObserver.open(
             this.propertyChangeCallback_.bind(this, 'settings.' + key));
       }
@@ skipping 22 matching lines @@
     };
 
     // ChromeVox is enbaled/disabled via the 'settings.accessibility' boolean
     // pref.
     this.settings.accessibility = false;
 
     // TODO(jlklein): Actually pull the data out of prefs and initialize.
   },
 
   /**
+   * Initializes some defaults for the downloads settings.
+   * @private
+   */
+  initializeDownloads_: function() {
+    this.settings.download = {
+      downloadLocation: '',
+      promptForDownload: false,
+    };
+  },
+
+  /**
    * @param {string} propertyPath The path before the property names.
    * @param {!Array<string>} added An array of keys which were added.
    * @param {!Array<string>} removed An array of keys which were removed.
    * @param {!Array<string>} changed An array of keys of properties whose
    *     values changed.
    * @param {function(string) : *} getOldValueFn A function which takes a
    *     property name and returns the old value for that property.
    * @private
    */
   propertyChangeCallback_: function(
       propertyPath, added, removed, changed, getOldValueFn) {
     Object.keys(changed).forEach(function(property) {
       console.log(
           `${propertyPath}.${property}`,
           `old : ${getOldValueFn(property)}`,
           `newValue : ${changed[property]}`);
 
       // TODO(jlklein): Actually set the changed property back to prefs.
     });
    },
 });