Move modules-dependent Event code out of core.
This is a follow-up of https://codereview.chromium.org/298893006/.

BUG=376506

Review URL: https://codereview.chromium.org/297103002

git-svn-id: svn://svn.chromium.org/blink/trunk@174888 bbb929c8-8fbe-4397-9dbb-9b2b20218538