Fenced frames: Add checks for fenced frame mapping in runAdAuction.

There are reported crashes happening when the pending urn gets erased
from the pending urn map. There are three possible causes:
1. The pending urn gets a double erase.
2. The generated urn is the same as a previous generated one.
3. The auction does not operate on the same urn mapping.

This CL added the CHECKs and replaced the DCHECKs with CHECKs for
those cases.

It is still unclear from the crash report why this is happening. We
are also unable to reproduce this locally. Given the number of crashes
is low, we will add those checks for now and it will help us determine
the root cause if the crashes happen again.

Bug: 1422301
Change-Id: Id0f54aae4cebf40b970a26a17a04c843be45ac7e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4324437
Commit-Queue: Matt Menke <mmenke@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Shivani Sharma <shivanisha@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1115339}