Run login capture code when a form or a formless password field is removed from...

Pages can use JS to handle logins instead of using form submission events and we should try to capture the saved values in those cases. One heuristic is if a password field is removed from a document.

We can likely use a custom DOM event (like bug 1132211) or Mutation Observers/Events for this but whatever we use will likely need to have access to the removed element and the associated username field before it gets deleted.

Some edge cases to consider:
* Do we want to capture if the user closes a tab? What about the page doing window.close (e.g. Persona)? These may also trigger the input to be removed when the document gets destroyed.
* We don't want to capture when the document gets deleted from the bfcache.
* We probably don't want to cause a second capture doorhanger to appear if the values were already captured via other methods (e.g. onbeforesubmit or other heuristics).
* We probably don't want to display multiple capture doorhangers (replacing the previous one) when there were multiple password fields in the form (e.g. a password change or registration form with double-entry) as the user may have already dismissed the first one and reopening would interrupt the user.