Add mstensho@opera.com to core/OWNERS
Add mstensho@opera.com to core/OWNERS

As discussed on core-owners-discuss@chromium.org

With great power comes great power bills, or something like that...
Go forth and LGTM!

Review URL: https://codereview.chromium.org/1131153004

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@195336 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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