Speculatively enable ProxyAuthDialogOnSigninScreen

Bug: 504928
Test: run locally
Change-Id: I74ffe767d95e2b47a5fbad87a4685bd31e01180f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1891312Reviewed-by: Denis Kuznetsov [CET] <antrim@chromium.org>
Commit-Queue: Roman Sorokin [CET] <rsorokin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711260}

chrome/browser/chromeos/login/wizard_controller_browsertest.cc

@@ -2038,9 +2038,8 @@ class WizardControllerProxyAuthOnSigninTest : public WizardControllerTest {
   DISALLOW_COPY_AND_ASSIGN(WizardControllerProxyAuthOnSigninTest);
 };
 
-// Disabled, see https://crbug.com/504928.
 IN_PROC_BROWSER_TEST_F(WizardControllerProxyAuthOnSigninTest,
-                       DISABLED_ProxyAuthDialogOnSigninScreen) {
+                       ProxyAuthDialogOnSigninScreen) {
   content::WindowedNotificationObserver auth_needed_waiter(
       chrome::NOTIFICATION_AUTH_NEEDED,
       content::NotificationService::AllSources());