[FedCM] Enable FedCmSkipWellKnownForSameSite by default

This skips the .well-known/web-identity checks if the RP and
IDP are in the same eTLD+1.

Approved by security and privacy in:
https://docs.google.com/document/d/1zTJVmW-zAm-pFeLIGV0IhInXVlLwSh9rz08E5QVy4NA/edit?resourcekey=0-LGYUJlxuj15i4phyz46nvQ

Bug: 1511767
Change-Id: I44ee15feb11354c4b58bfab74e69b7d4af97da87
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5177812
Reviewed-by: Nicolás Peña <npm@chromium.org>
Commit-Queue: Christian Biesinger <cbiesinger@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1244800}