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The top-level histograms.xml file is deprecated. This change
* Ensures that `set noparent` applies for all CLs that update
  histograms.
* Cleans up cruft from the //tools/metrics/histograms/OWNERS file.
* Leaves cthomp@ as a general metrics reviewer.
* Removes all other general metrics reviewers (see below).

Note that I have only included cthomp@ as a non-Metrics-team reviewer in
this new file. If you were previously listed as a metrics reviewer and
wish to continue to be a metrics reviewer, feel free to send a CL
re-adding yourself -- typically to one or several relevant
subdirectories' OWNERS files, but it's also fine to add yourself as an
owner for the entire histograms_xml directory if you prefer.

If you were not previously trained as a metrics reviewer but would like
to take on stewardship for some metrics reviews, please reach out to the
Metrics team. We'll be more than happy to get you onboarded!

R=sweilun

Fixed: 1133371a
Change-Id: I2d3207e74114eead80dc753c5e9f4aa81785ec99
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2462166
Commit-Queue: Weilun Shi <sweilun@chromium.org>
Reviewed-by: Weilun Shi <sweilun@chromium.org>
Auto-Submit: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816476}