Make EventListener a callback interface

Make EventListener a callback interface instead of a regular interface
to match the specification:
http://dom.spec.whatwg.org/#eventlistener

This currently has no impact on the generated code because the
addEventListener() / removeEventListener() methods are special-cased
in the bindings generator.

R=haraken

Review URL: https://chromiumcodereview.appspot.com/22553002

git-svn-id: svn://svn.chromium.org/blink/trunk@155676 bbb929c8-8fbe-4397-9dbb-9b2b20218538