When this is fixed then the description of the "Has prefetched pages" entry in the OfflinePagesPrefetchUsage metrics enum should also be updated to indicate when this bucket is reported to.

We chose to remove this bucket instead of reporting to it. The value of the information it would provide doesn't seem to justify the complexities of the compromise of it being reported: impacting resource usage by forcing the OP metadata DB to be opened at least once per day versus the consistency of its reporting.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d0f379fd28d7e266b921cb89e77fe58e67d9e378

commit d0f379fd28d7e266b921cb89e77fe58e67d9e378
Author: Carlos Knippschild <carlosk@chromium.org>
Date: Wed Aug 01 21:22:59 2018

Removed unused bucket of OfflinePages.PrefetchUsage

The "has pages" bucket of the OfflinePages.PrefetchUsage histogram never
had data collected into it. We finally decided to fully remove it as the
information it would provide doesn't seem to justify the extra costs and
changes associated with adding the proper trigger to report it.

Bug:  860830 
Change-Id: I837e5f4e6184175841773450e44eaedde94513e3
Reviewed-on: https://chromium-review.googlesource.com/1157281
Reviewed-by: Dmitry Titov <dimich@chromium.org>
Reviewed-by: Ramin Halavati <rhalavati@chromium.org>
Commit-Queue: Carlos Knippschild <carlosk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579945}
[modify] https://crrev.com/d0f379fd28d7e266b921cb89e77fe58e67d9e378/chrome/browser/offline_pages/offline_page_tab_helper_unittest.cc
[modify] https://crrev.com/d0f379fd28d7e266b921cb89e77fe58e67d9e378/chrome/browser/offline_pages/prefetch/offline_metrics_collector_impl.cc
[modify] https://crrev.com/d0f379fd28d7e266b921cb89e77fe58e67d9e378/chrome/browser/offline_pages/prefetch/offline_metrics_collector_impl.h
[modify] https://crrev.com/d0f379fd28d7e266b921cb89e77fe58e67d9e378/chrome/browser/offline_pages/prefetch/offline_metrics_collector_impl_unittest.cc
[modify] https://crrev.com/d0f379fd28d7e266b921cb89e77fe58e67d9e378/chrome/browser/prefs/pref_service_incognito_whitelist.cc
[modify] https://crrev.com/d0f379fd28d7e266b921cb89e77fe58e67d9e378/chrome/common/pref_names.cc
[modify] https://crrev.com/d0f379fd28d7e266b921cb89e77fe58e67d9e378/chrome/common/pref_names.h
[modify] https://crrev.com/d0f379fd28d7e266b921cb89e77fe58e67d9e378/components/offline_pages/core/prefetch/offline_metrics_collector.h
[modify] https://crrev.com/d0f379fd28d7e266b921cb89e77fe58e67d9e378/components/offline_pages/core/prefetch/test_offline_metrics_collector.h
[modify] https://crrev.com/d0f379fd28d7e266b921cb89e77fe58e67d9e378/tools/metrics/histograms/enums.xml