Move histograms.xml OWNERS file into the histograms_xml directory

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}

tools/metrics/histograms/OWNERS

-# Core Metrics Owners
-#
-# Note: Please send CLs to chromium-metrics-reviews@google.com rather than to
-# specific individuals from the list of core metrics owners. These CLs will be
-# automatically reassigned to a reviewer within about 5 minutes. This approach
-# helps our team to load-balance incoming reviews. Googlers can read more about
-# this at go/gwsq-gerrit.
-per-file histograms.xml=file://base/metrics/OWNERS
 # Extending enums by adding new buckets is allowed without OWNERS review, but
-# you should add a metrics reviewer if you are changing the semantics of existing
-# buckets.
+# you should add a metrics reviewer if you are changing the semantics of
+# existing buckets.
 per-file enums.xml=*
 
-# Prefer chromium-metrics-reviews@google.com instead.
-# Use the following owners only if:
-#  - You work in the same area as them.
-#  - They are already a good reviewer for the non-metrics part of the CL.
-per-file histograms.xml=alexilin@chromium.org
-per-file histograms.xml=altimin@chromium.org # Scheduling-related metrics.
-per-file histograms.xml=csharrison@chromium.org
-per-file histograms.xml=cthomp@chromium.org  # For security-related metrics only.
-per-file histograms.xml=dschinazi@chromium.org  # For networking-related metrics only.
-per-file histograms.xml=schenney@chromium.org
-per-file histograms.xml=tbansal@chromium.org
-per-file histograms.xml=tby@chromium.org # For chrome knowledge team metrics only.
-per-file histograms.xml=chrome-metrics-team+robot@google.com # For bot updates to expiry.
+# The top-level histograms.xml file is deprecated and should not be modified.
+# It's temporarily still present because some tools have the filename hardcoded.
+per-file histograms.xml=file://base/metrics/OWNERS
 per-file histograms.xml=set noparent

tools/metrics/histograms/histograms_xml/OWNERS

+# Core Metrics Owners
+#
+# Note: Please send CLs to chromium-metrics-reviews@google.com rather than to
+# specific individuals from the list of core metrics owners. These CLs will be
+# automatically reassigned to a reviewer within about 5 minutes. This approach
+# helps our team to load-balance incoming reviews. Googlers can read more about
+# this at go/gwsq-gerrit.
+file://base/metrics/OWNERS
+
+# Prefer chromium-metrics-reviews@google.com instead.
+# Use the following owners only if:
+#  - You work in the same area as them.
+#  - They are already a good reviewer for the non-metrics part of the CL.
+cthomp@chromium.org  # For security-related metrics only.
+
+chrome-metrics-team+robot@google.com # For bot updates to expiry.
+
+# All metrics changes should be reviewed by metrics reviewers.
+set noparent