webauthn: don't restart requests after non-automatic dispatch to iCKC.

The `priority_mechanisms_index_` member was not ephemeral state, but
should have been. Without this change, if iCKC was auto-dispatched
initially, every time the user cancels it we'll restart the request, but
that should only happen once.

Bug: 1439987
Change-Id: I4298a0dc5d98863565fdb9c6d27ad7599455f5c7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4808927


Auto-Submit: Adam Langley <agl@chromium.org>
Commit-Queue: Adam Langley <agl@chromium.org>
Reviewed-by: Nina Satragno <nsatragno@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1187926}