[FedCM] Add a helper function to check for same-site

We currently have two places in the code that check if two origins are
same-site; add a helper function instead and use it from both places.

This will also be useful when we support same-site login status setting.

Note that the trustworthy origin check is safe to remove because we
already ensure that the IDP origin is trustworthy.

R=yigu@chromium.org

Bug: 1516640
Change-Id: I0545fa388179bb03372a806f8f5f86ebeeb92ce5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5200179
Commit-Queue: Christian Biesinger <cbiesinger@chromium.org>
Reviewed-by: Yi Gu <yigu@chromium.org>
Auto-Submit: Christian Biesinger <cbiesinger@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1248357}