Document how to make a class implement EventTarget

R=esprehn@chromium.org,haraken@chromium.org,nbarth@chromium.org

Review URL: https://codereview.chromium.org/402453002

git-svn-id: svn://svn.chromium.org/blink/trunk@178323 bbb929c8-8fbe-4397-9dbb-9b2b20218538