Add ager@chromium.org to core/OWNERS

TBR=eseidel@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/307933012

git-svn-id: svn://svn.chromium.org/blink/trunk@175254 bbb929c8-8fbe-4397-9dbb-9b2b20218538