Add rbyers to core/OWNERS.

BUG=none
R=blink-core-owners
TBR=eseidel@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/277783003

git-svn-id: svn://svn.chromium.org/blink/trunk@174011 bbb929c8-8fbe-4397-9dbb-9b2b20218538