[FedCM] Skip showing iframe domain

Showing iframe domain could be confusing to users. This patch
makes the minimal change to fix the issue for an easier merge.
Will clean up the logic as a follow up.

Change-Id: I022e55f9d13fdac9f3e0d561c9ac4996618a6f15
Bug: 1418719
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4560225


Commit-Queue: Yi Gu <yigu@chromium.org>
Reviewed-by: Zachary Tan <tanzachary@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1148689}