[WebAuthn] Stash GPM PIN during initial security domain service enroll

When a user create a GPM PIN while adding a device with existing
security domain secrets, we stash the PIN for use in the ensuing
transaction.

If there are no existing security domain secrets and a GPM PIN is
being created, we don't stash the PIN. The code CHECKs on an empty
PIN after registration when it attempts to proceed with the
transaction.

Instead, stash on both instances.

Change-Id: I52ad89ec4972600bd8113e81089fff7f6dbc5fd4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5405476
Commit-Queue: Ken Buchanan <kenrb@chromium.org>
Auto-Submit: Ken Buchanan <kenrb@chromium.org>
Commit-Queue: Adam Langley <agl@chromium.org>
Reviewed-by: Adam Langley <agl@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1279888}