Remove some special-casing of EventListener

No need to make the bindings use a raw ptr for removeEventListener;
we can just call .get() in the implementation.

Also make the bindings code a bit more generic by not assuming that the
event listener is the second argument.

BUG=383699

Review URL: https://codereview.chromium.org/383873002

git-svn-id: svn://svn.chromium.org/blink/trunk@178318 bbb929c8-8fbe-4397-9dbb-9b2b20218538