Add kron as owner of api/uma_metrics.h

kron is an owner of UMA metrics for WebRTC in chromium,
see tools/metrics/histograms/histograms_xml/web_rtc/OWNERS

Bug: webrtc:12096
Change-Id: I9804d747fc4e52d2ed2a9d96cc4ed315639210da
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/221961
Reviewed-by: Harald Alvestrand <[email protected]>
Commit-Queue: Johannes Kron <[email protected]>
Cr-Commit-Position: refs/heads/master@{#34268}

api/OWNERS

@@ -11,15 +11,4 @@ per-file peer_connection*[email protected]
 
 per-file [email protected]
 
-# Please keep this list in sync with Chromium's //base/metrics/OWNERS and
-# send a CL when you notice any difference.
-# Even if people in the list below cannot formally grant +1 on WebRTC, it
-# is good to get their LGTM before sending the CL to one of the folder OWNERS.
-per-file [email protected]
-per-file [email protected]
-per-file [email protected]
-per-file [email protected]
-per-file [email protected]
-per-file [email protected]
-per-file [email protected]
-per-file [email protected]
+per-file [email protected]