While adding a new Net.HttpResponse code metric to the PageLoad UKM event [1], bmcquade@ helped us figure out that PageLoad events aren't logged for error response codes (e.g., 404s/500s) [2].

After discussing with bmcquade@, we determined that we could get the metrics we need for now without further changes, but other consumers of PageLoad metrics might find it useful to have them in the future. This would require some refactoring of the page load metrics code however (i.e., allowing error pages to be logged, but only for some metrics that are relevant; passing NavigationHandle to the appropriate callbacks), so this bug is for tracking that potential followup work.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/1238471
[2] https://cs.chromium.org/chromium/src/chrome/common/page_load_metrics/page_track_decider.cc?sq=package:chromium&g=0&l=27