Adding "observing event listeners" section to DOM spec

We've discussed passive event monitoring (EventListenerOptions
<https://github.com/rbyers/EventListenerOptions>, scroll-blocks-on, etc.)
in this groups in the past.  Formalizing that requires acknowledging that
user agents sometimes observe the presence of event listeners.  I've been
working with @annevk on some DOM spec text for this
<https://rawgit.com/RByers/dom/event-listener-options/dom.html#observing-event-listeners>,
and hope to eventually add something to the TouchEvents spec
<https://github.com/w3c/touch-events/issues/6#issuecomment-142364472> to
define it's usage explicitly.

I'd appreciate any feedback you have on this (ideally here
<https://github.com/RByers/EventListenerOptions/issues/20>).

Thanks,
   Rick