Move dom/*Event* to events to match the DOM vs. Events spec separation

This is not quite perfect, but it does make the DOM directory much
smaller and thus hopefully much easier to reason about.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/23983049

git-svn-id: svn://svn.chromium.org/blink/trunk@158219 bbb929c8-8fbe-4397-9dbb-9b2b20218538