Fix flaky GuestErrorScreenTest by adding RestartWaiter

Start listening for restart event before clicking on the guest
signin button.

Bug: 963123
Change-Id: Ie28aacac3ccfa675ab194c2f497ca6d0a67dadb3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1612380
Commit-Queue: Tony De Luna <tonydeluna@chromium.org>
Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659718}

chrome/browser/chromeos/login/error_screen_browsertest.cc

@@ -203,11 +203,12 @@ IN_PROC_BROWSER_TEST_F(GuestErrorScreenTest, PRE_GuestLogin) {
 
   OobeScreenWaiter(ErrorScreenView::kScreenId).Wait();
   test::OobeJS().ExpectVisiblePath({"error-guest-signin-link"});
-  test::OobeJS().ClickOnPath({"error-guest-signin-link"});
 
   base::RunLoop restart_job_waiter;
   FakeSessionManagerClient::Get()->set_restart_job_callback(
       restart_job_waiter.QuitClosure());
+
+  test::OobeJS().ClickOnPath({"error-guest-signin-link"});
   restart_job_waiter.Run();
 }