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

chrome/browser/resources/chromeos/login/oobe_buttons.js

Index: chrome/browser/resources/chromeos/login/oobe_buttons.js
diff --git a/chrome/browser/resources/chromeos/login/oobe_buttons.js b/chrome/browser/resources/chromeos/login/oobe_buttons.js
index 1b469a99bcabbbdb2a007c844a33255f6d158e7d..b032445bc29d8e36f57fb8acb953acdb46350d0f 100644
--- a/chrome/browser/resources/chromeos/login/oobe_buttons.js
+++ b/chrome/browser/resources/chromeos/login/oobe_buttons.js
@@ -22,6 +22,23 @@ Polymer({
 });
 
 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: {