Fire a11y alert on smart card login failure

Trigger the a11y alert that reads the "Couldn’t recognize your smart
card" message when the challenge-response login (a.k.a. smart card
login) fails.

Bug: 1004449
Test: add a smart card user onto Chromebook, remove the smart card, enable screen reader, click "Start" ("->"), check that "Couldn’t recognize your smart card" is read
Change-Id: Iea19f5d8a3dd00adb1729d4c32879e1d3678890a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2022952Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Maksim Ivanov <emaxx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#735519}

ash/login/ui/login_auth_user_view.cc

@@ -612,6 +612,11 @@ class LoginAuthUserView::ChallengeResponseView : public views::View,
     icon_->SetImage(GetImageForIcon());
     label_->SetText(GetTextForLabel());
 
+    if (state == State::kFailure) {
+      label_->NotifyAccessibilityEvent(ax::mojom::Event::kAlert,
+                                       /*send_native_event=*/true);
+    }
+
     Layout();
   }