Which ones?

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/965aedb472c74825292118de14decf49edde4bb7

commit 965aedb472c74825292118de14decf49edde4bb7
Author: Ryan Sturm <ryansturm@chromium.org>
Date: Thu Jun 14 19:41:18 2018

Removing obsolete previews histograms in opt out store

These histograms haven't been viewed, and don't provide much value.

Bug:  852621 
Change-Id: Id3c859b9ec7fe0118010c1c92e787df8773f2090
Reviewed-on: https://chromium-review.googlesource.com/1100176
Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Commit-Queue: Ryan Sturm <ryansturm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567363}
[modify] https://crrev.com/965aedb472c74825292118de14decf49edde4bb7/components/previews/core/previews_opt_out_store_sql.cc
[modify] https://crrev.com/965aedb472c74825292118de14decf49edde4bb7/components/previews/core/previews_opt_out_store_sql_unittest.cc
[modify] https://crrev.com/965aedb472c74825292118de14decf49edde4bb7/tools/metrics/histograms/histograms.xml