ChromeOS OOBE: Implement network selection screen "Back" button.

chrome/browser/resources/chromeos/login/oobe_welcome.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.
 
 /**
  * @fileoverview Polymer element for displaying material design OOBE.
  */
 
 Polymer({
   is: 'oobe-welcome-md',
@@ skipping 105 matching lines @@
   },
 
   /**
    * Returns custom items for network selector.
    */
   _getNetworkCustomItems: function() {
     var self = this;
     return [
       {
         customItemName: 'proxySettingsMenuName',
-        polymerIcon: 'oobe-welcome:add',
+        polymerIcon: 'oobe-welcome-20:add-proxy',
         customData: {
           onTap: function() { self.OpenProxySettingsDialog_(); },
         },
       },
       {
         customItemName: 'addWiFiNetworkMenuName',
-        polymerIcon: 'oobe-welcome:add',
+        polymerIcon: 'oobe-welcome-20:add-wifi',
         customData: {
           onTap: function() { self.OpenAddWiFiNetworkDialog_(); },
         },
       },
       {
         customItemName: 'addMobileNetworkMenuName',
-        polymerIcon: 'oobe-welcome:add',
+        polymerIcon: 'oobe-welcome-20:add-cellular',
         customData: {
           onTap: function() { self.OpenAddWiFiNetworkDialog_(); },
         },
       },
     ];
   },
 
   /**
    * Handle "Next" button for "Welcome" screen.
    *
@@ skipping 119 matching lines @@
   /**
    * @param {!Event} event
    * @private
    */
   onNetworkListCustomItemSelected_: function(e) {
     var itemState = e.detail;
     itemState.customData.onTap();
   },
 
   /**
+   * Handle "<- Back" button on network selection screen.
+   *
+   * @private
+   */
+  onNetworkSelectionBackButtonPressed_: function() {
+    this.networkLastSelectedGuid_ = '';
+    this.hideAllScreens_();
+    this.welcomeScreenShown = true;
+  },
+
+  /**
    * Handle language selection.
    *
    * @param {!{detail: {!OobeTypes.LanguageDsc}}} event
    * @private
    */
   onLanguageSelected_: function(event) {
     var item = event.detail;
     var languageId = item.value;
     this.screen.onLanguageSelected_(languageId);
   },
@@ skipping 38 matching lines @@
    * containing Chromium callback name.
    *
    * @private
    * @param {!Event} event
    */
   onA11yOptionChanged_: function(event) {
     chrome.send(
         event.currentTarget.chromeMessage, [event.currentTarget.checked]);
   },
 });