[iOS][EG] Update testRepostFormCancelling for iOS 26 alert behavior

This change updates the testRepostFormCancelling egtest to handle
changes to UIAlertController in the iOS 26 SDK.

In iOS 26, the form repost confirmation alert no longer has a 'Cancel'
button, only a 'Continue' button. This change updates the test to use
dismissByTappingOnTheWindowOfPopover: on iOS 26+ to properly dismiss the
alert, while preserving the old logic for previous OS versions.

Fixed: 432002584
Change-Id: Ib998501928dd9a08c2f90544124c4e88755c7561
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6766104
Commit-Queue: Benjamin Williams <bwwilliams@google.com>
Reviewed-by: Sergio Collazos <sczs@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1488482}