Apparently this is somewhat working as intended. So just marking as won't fix.

actually going to do this

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ea5bd633c61ae2f52e9774fed043e76c9ff23d4a

commit ea5bd633c61ae2f52e9774fed043e76c9ff23d4a
Author: Sophie Chang <sophiechang@chromium.org>
Date: Thu Jan 17 00:46:30 2019

Start recording Previews.PageEndReason.Offline

The path is slightly different than other previews and does not call in the previews_ukm_observer for some reason.

Bug:  921091 
Change-Id: I31e55485c88919563bbe2d842ee4a6b25026bbe3
Reviewed-on: https://chromium-review.googlesource.com/c/1414190
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Commit-Queue: Sophie Chang <sophiechang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623466}
[modify] https://crrev.com/ea5bd633c61ae2f52e9774fed043e76c9ff23d4a/chrome/browser/page_load_metrics/observers/offline_page_previews_page_load_metrics_observer.cc
[modify] https://crrev.com/ea5bd633c61ae2f52e9774fed043e76c9ff23d4a/chrome/browser/page_load_metrics/observers/offline_page_previews_page_load_metrics_observer.h
[modify] https://crrev.com/ea5bd633c61ae2f52e9774fed043e76c9ff23d4a/chrome/browser/page_load_metrics/observers/offline_page_previews_page_load_metrics_observer_unittest.cc