Initialize 'cancelable' to false in Event::new_inherited. Fixes #3855.
Remove ./dom/nodes/Document-createEvent.html.ini in tests/wpt/metadata because we expect all tests to pass now. Fixes #3855
Simplify the implementation of NodeHelpers::child_elements.
Simplify the implementation of NodeIterator::next_child.
auto merge of #3870 : mvanderh/servo/master, r=Ms2ger

Fixes #3855.

I'm not sure if this is the right place to set `cancelable`. If changing the default value in `Event::new_inherited` is not satisfactory, it will be possible to set the `Cell`'s value to `false` in `Event::new` itself.
auto merge of #3871 : Ms2ger/servo/node-cleanup, r=Ms2ger
Corral debug command-line options into a --debug/-Z flag

This should make help output a lot cleaner and simplify the way that
uncommon debug options are passed.