Update owners for webrtc_cases

Change-Id: Idb272f9406080f1c1fac2c70d7538400ea115311
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2569815
Commit-Queue: John Chen <johnchen@chromium.org>
Reviewed-by: John Chen <johnchen@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Mirko Bonadei <mbonadei@chromium.org>
Auto-Submit: Christoffer Jansson <jansson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836145}

tools/perf/page_sets/OWNERS

 per-file update_webrtc_cases=ehmaldonado@chromium.org
-per-file update_webrtc_cases=jansson@chromium.org
+per-file update_webrtc_cases=titovartem@chromium.org
+per-file update_webrtc_cases=mbonadei@chromium.org
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