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

chrome/browser/resources/chromeos/login/oobe_buttons.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.
 
 Polymer({
   is: 'oobe-text-button',
 
   properties: {
     disabled: {type: Boolean, value: false, reflectToAttribute: true},
 
     inverse: Boolean,
   },
 
   focus: function() {
     this.$.textButton.focus();
   },
 
   onClick_: function(e) {
     if (this.disabled)
       e.stopPropagation();
   }
 });
 
 Polymer({
+  is: 'oobe-back-button',
+
+  properties: {
+    disabled: {type: Boolean, value: false, reflectToAttribute: true},
+  },
+
+  focus: function() {
+    this.$.button.focus();
+  },
+
+  onClick_: function(e) {
+    if (this.disabled)
+      e.stopPropagation();
+  }
+});
+
+Polymer({
   is: 'oobe-welcome-secondary-button',
 
   properties: {
     icon: String,
 
     ariaLabel: String
   },
 });