Move modules-dependent eventtarget code out of core.
This is a follow-up CL on https://codereview.chromium.org/275283002/

BUG=371581

Review URL: https://codereview.chromium.org/297493004

git-svn-id: svn://svn.chromium.org/blink/trunk@174387 bbb929c8-8fbe-4397-9dbb-9b2b20218538