webauthn: add tests for the new pop-up window UI steps.

Change-Id: I3ce5ef35fb91f1c97732f88cd1c713e014a55a17
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5386677


Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Commit-Queue: Adam Langley <agl@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1279998}