Fix counting passkeys in SavedInfoHandler

The implementation for displaying password and passkey counts on the
"Your saved info" settings page had a bug where the passkey count
would appear as 0.

This was because `SavedPasswordsPresenter::GetSavedCredentials()`
does not include passkeys and `passkey_credential_id` field can't
be reliably used, despite the comment in the code.
Passkeys are now managed separately by the `webauthn::PasskeyModel`
and this is used in the new approach.

Both password and passkey counts are now accurately fetched.

Bug: 448352004
Change-Id: I2634b8ece8eb7661286971393bbeb6b06938bce1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7015130
Reviewed-by: Stanislav Mikheyev <mikhe@google.com>
Reviewed-by: Vasilii Sukhanov <vasilii@chromium.org>
Commit-Queue: Ireneusz Szulc <ireneuszszulc@google.com>
Cr-Commit-Position: refs/heads/main@{#1526788}