Expose events in workers as per latest DOM specification

Expose events in workers as per latest DOM specification:
https://github.com/whatwg/dom/commit/0f69e7175dcae9edffa636bb213991330d225bf3

This CL exposes Event, CustomEvent and EventTarget to Workers and not only
in the primary global scope (Window), as per the latest DOM specification.

Firefox 29 exposes both Event and EventTarget to workers but not CustomEvent
(known bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1003432).
IE11 exposes Event only to workers.

R=arv@chromium.org, tkent@chromium.org
BUG=370011
TEST=webexposed/global-constructors-listing-dedicated-worker.html
TEST=webexposed/global-constructors-listing-shared-worker.html

Review URL: https://codereview.chromium.org/262173006

git-svn-id: svn://svn.chromium.org/blink/trunk@173328 bbb929c8-8fbe-4397-9dbb-9b2b20218538