Add comment to cthomp entry in tools/metrics/histograms/OWNERS

Specify the relevant metrics cthomp reviews in the OWNERS file, on the
same line so that `git cl owners` and similar tools will include it.

Change-Id: I37769bd88edadf77dce56f23da0002eef0546b73
Reviewed-on: https://chromium-review.googlesource.com/c/1453082
Auto-Submit: Christopher Thompson <cthomp@chromium.org>
Commit-Queue: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630009}

tools/metrics/histograms/OWNERS

@@ -7,7 +7,7 @@ per-file histograms.xml=file://base/metrics/OWNERS
 
 per-file histograms.xml=alexilin@chromium.org
 per-file histograms.xml=csharrison@chromium.org
-per-file histograms.xml=cthomp@chromium.org
+per-file histograms.xml=cthomp@chromium.org  # For security-related metrics only.
 per-file histograms.xml=schenney@chromium.org
 
 per-file histograms.xml=set noparent