[crd host] Add test to verify source info for disconnect event

dcheng@ and I had chatted about crbug.com/41342136 in chromium slack
channel, and his suggestion was to just add a test to verify that the
source info is properly populated then call it a day. The bug is stalled
since

names can't be removed without getting them fixed. Adding a test can
ensure we won't be caught off guard if they ever move forward with that
bug.

base: :Location has other non-task annotation usages so the function
Change-Id: Ifc9485aae270ad0eaf649df5306eb665c96057ff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6354086
Auto-Submit: Yuwei Huang <yuweih@chromium.org>
Commit-Queue: Joe Downing <joedow@chromium.org>
Reviewed-by: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1432663}