webauthn: create enclave discovery outside of transport-based loop.

The enclave pretends to have "internal" transport, but it's not really a
platform authenticator. Since its discovery was created when "internal"
discoveries were created, it wasn't getting created on Windows when
webauthn.dll was present as that causes "internal" to be removed from
the list of active transports.

ef577965ace59598eb3e5960555301bf0175f053 tried to fix this, but having
the "internal" transport in Windows requests causes "This device" to
appear as a mechanism.

Bug: 339284461
Change-Id: I4f9505e69739de2d81f2dcf886644a2c4281e2f0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5523820
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Commit-Queue: Adam Langley <agl@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1298431}