Initially this was going to be a simple cleanup: Remove some useless namespaces
here and there and so on, remove `using` statements from the header and so on.
But unfortunately, DOMIntersectionObserver.h (which is included in Element.h,
unnecessarily) ended up exposing `Element` unnamespaced to a lot of code, so I
had to fix that.
Differential Revision: https://phabricator.services.mozilla.com/D55316
--HG--
extra : moz-landing-system : lando
