[FedCM] Show warning in console if mediation is used in the wrong place

While almost all FedCM related parameters live under `identity`,
`mediation` does not since it's defined in the upstream CredMan API.

We noticed that such exception brought confusion to developers so we are
adding console logs when `mediation` is used in the wrong place.

Bug: 358379366
Change-Id: I82b0d02444020f7975768d800dc9fa4d041d6fb2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5774004


Reviewed-by: Nicolás Peña <npm@chromium.org>
Commit-Queue: Yi Gu <yigu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1339631}