Add OWNERS for net histograms

We currently have tools/metrics/histograms/OWNERS set up so that
Net experts can approve changes to Net. Now that the split at
https://crrev.com/c/2428796 has landed, we'd like to retain
that approval process, so this CL adds a new OWNERS file.

Change-Id: I5f0fc1c9ef473e01d4907a7861d0bf4a5840bd24
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2436556
Auto-Submit: David Schinazi <dschinazi@chromium.org>
Reviewed-by: Weilun Shi <sweilun@chromium.org>
Reviewed-by: David Schinazi <dschinazi@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811544}

tools/metrics/histograms/histograms_xml/net/OWNERS

+# Networking Metrics Owners
+
+dschinazi@chromium.org
+eroman@chromium.org
+
+# If none of these reviewers are available, please send CLs to
+# <chromium-metrics-reviews@google.com>. See tools/metrics/histograms/OWNERS
+# for details.
+
+# COMPONENT: Internals>Metrics