Add unit test for the back button in the new GAIA.

google_apis/test/embedded_setup_chromeos.html

Index: google_apis/test/embedded_setup_chromeos.html
diff --git a/google_apis/test/embedded_setup_chromeos.html b/google_apis/test/embedded_setup_chromeos.html
index d2175f00336984473505f1dc0c5ccd946e5ece05..082c9ecc15d252d99450cba15c776c652c48e6f4 100644
--- a/google_apis/test/embedded_setup_chromeos.html
+++ b/google_apis/test/embedded_setup_chromeos.html
@@ -21,6 +21,7 @@ gaia.chromeOSLogin.registerHtml5Listener = function() {
     }
   };
   window.addEventListener('message', onMessage);
+  window.addEventListener("popstate", function(e) { goBack(); });
 }
 
 gaia.chromeOSLogin.clearOldAttempts = function() {
@@ -50,6 +51,19 @@ gaia.chromeOSLogin.backButton = function(show) {
       gaia.chromeOSLogin.parent_webview_url_);
 };
 
+function goFirstPage() {
+  document.getElementById('page1').hidden = false;
+  document.getElementById('page2').hidden = true;
+  history.replaceState({}, '', window.location.pathname + '#identifier');
+  gaia.chromeOSLogin.backButton(false);
+}
+
+function goBack() {
+  if (!document.getElementById('page2').hidden) {
+    goFirstPage();
+  }
+}
+
 function goNext() {
   if (!document.getElementById("page1").hidden) {
     document.getElementById("page1").hidden = true;
@@ -88,8 +102,7 @@ function goNext() {
 
 function onLoad() {
   gaia.chromeOSLogin.registerHtml5Listener();
-  document.getElementById("page1").hidden = false;
-  history.replaceState({}, "", window.location.pathname + "#identifier");
+  goFirstPage();
   gaia.chromeOSLogin.clearOldAttempts();
 }