Add EMEA-based reviewers for UseCounter histogram changes.

We're TBRing UseCounter changes to Ilya, et al. more often than I'd
like. This patch adds Jochen and I as EMEA-based reviewers for these
mechanical changes until such time as the mechanically-generated
UseCounter data is split out into a file with broader ownership.

R=jochen@chromium.org,isherman@chromium.org,asvitkine@chromium.org

Review-Url: https://codereview.chromium.org/2168463005
Cr-Commit-Position: refs/heads/master@{#406670}

tools/metrics/histograms/OWNERS

 # Metrics changes should always be reviewed by owners.
-per-file histograms.xml=file://tools/metrics/OWNERS 
 per-file histograms.xml=set noparent
+per-file histograms.xml=file://tools/metrics/OWNERS
+
+# EMEA-based reviewers for UseCounter histogram changes only.
+per-file histograms.xml=jochen@chromium.org
+per-file histograms.xml=mkwst@chromium.org