[iOS] Add missing PasswordManagerClientBridge function implementation.

A previous change implemented the ability to handle logins with existing
credentials, but it wasn't implemented in the CWVAutofillController.
This change makes implements the function to correct a crash that was
caused by the missing implementation.

Previous change:
https://chromium-review.googlesource.com/c/chromium/src/+/6455406

Change-Id: I14cc84c928554b2bbe94cd5d1746361cf44eda87
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6581743
Commit-Queue: Carlos Acosta <cyacosta@google.com>
Reviewed-by: John Wu <jzw@chromium.org>
Reviewed-by: Hiroshi Ichikawa <ichikawa@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1466404}
diff --git a/ios/web_view/internal/autofill/cwv_autofill_controller.mm b/ios/web_view/internal/autofill/cwv_autofill_controller.mm
index 3c54ca83..b895b8a 100644
--- a/ios/web_view/internal/autofill/cwv_autofill_controller.mm
+++ b/ios/web_view/internal/autofill/cwv_autofill_controller.mm
@@ -783,6 +783,10 @@
   // No op
 }
 
+- (void)showSignedInWithSavedCredentialMessage {
+  [self didLoginWithExistingPassword];
+}
+
 - (void)didLoginWithExistingPassword {
   if ([self.delegate respondsToSelector:@selector
                      (autofillControllerDidLoginWithExistingPassword:)]) {