Focus email input after back action on Dent flows

Focus email input after back button action on EDU and Inline Login
sign-in flows.

Bug: 1004243
Change-Id: I8021da9d11296040416da2d48a69d0dc136620a2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2207452Reviewed-by: Kush Sinha <sinhak@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Andrei Salavei <solovey@google.com>
Cr-Commit-Position: refs/heads/master@{#771559}

chrome/browser/resources/chromeos/edu_login/edu_login_signin.js

@@ -153,6 +153,7 @@ Polymer({
   navigateBackInWebview_() {
     if (this.$.signinFrame.canGoBack()) {
       this.$.signinFrame.back();
+      this.$.signinFrame.focus();
     } else {
       // Reload the webview. It allows users to go back and try to add another
       // account if something goes wrong in the webview (e.g. SAML server

chrome/browser/resources/inline_login/inline_login.js

@@ -148,6 +148,7 @@ cr.define('inline.login', function() {
   function navigateBackInWebview() {
     if ($('signin-frame').canGoBack()) {
       $('signin-frame').back();
+      $('signin-frame').focus();
     } else {
       closeDialog();
     }