[Web Payment] Secure payment confirmation UI polish.

Before this patch, if WebAuthn returned an error when creating an
assertion, Web Payment called into ShowErrorMessage(), which called into
OnCancel(), which rejected the PaymentRequest.show() promise, but kept
the secure payment confirmation UI open.

This patch makes sure that the dialog is closed in OnCancel().

After this patch, if WebAuthn returns an error, then the secure payment
confirmation UI is closed.

Bug: 1122696
Change-Id: I99fae1d7b0b903338fca8cd03ecf57646dd78699
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2380550
Commit-Queue: Rouslan Solomakhin <rouslan@chromium.org>
Commit-Queue: Nick Burris <nburris@chromium.org>
Auto-Submit: Rouslan Solomakhin <rouslan@chromium.org>
Reviewed-by: Nick Burris <nburris@chromium.org>
Cr-Commit-Position: refs/heads/master@{#802349}

components/payments/content/secure_payment_confirmation_controller.cc

@@ -165,6 +165,8 @@ void SecurePaymentConfirmationController::OnDismiss() {
 }
 
 void SecurePaymentConfirmationController::OnCancel() {
+  CloseDialog();
+
   if (!request_)
     return;