Use EventHandler type instead of EventListener for event handler attributes

Distinguish in the IDL the HTML EventHandler attributes and DOM EventListener
types added via addEventListener() method to match the specifications:
http://www.whatwg.org/specs/web-apps/current-work/multipage/webappapis.html#eventhandler
http://dom.spec.whatwg.org/#eventlistener

This makes our IDL closer to the specifications but there is no web-exposed
behavior change with this patch. In the future, however, we may consider
supporting only JS Functions (not objects with handleEvent() method) as
input for EventHandler attributes to match the specification.

R=haraken

Review URL: https://chromiumcodereview.appspot.com/22581002

git-svn-id: svn://svn.chromium.org/blink/trunk@155702 bbb929c8-8fbe-4397-9dbb-9b2b20218538