Cleanup Page::remove.
Remove comment that hasn't been true since Document became a Node.

The panic was removed in 99a36cbeb6077976861d94b7af16e9e57994a14d.
Cleanup PageIterator::next.
auto merge of #4315 : Ms2ger/servo/page, r=jdm
ports/cef: Implement accelerated compositing for the CEF port.