Add Tests for overridden user agents within debug reporting.

This is a test only change.

This tests that when we override a user agent, that it is accurately
reflected within the debug reporting requests.

This also changes the names of the tests to better reflect the
feature flag.

Change-Id: Icf4c1b201608d4d6cff34056a46cce5d43784701
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5957123
Reviewed-by: Maks Orlovich <morlovich@chromium.org>
Commit-Queue: Youssef Bourouphael <ybourouphael@google.com>
Reviewed-by: Orr Bernstein <orrb@google.com>
Cr-Commit-Position: refs/heads/main@{#1374798}