s/AbstractNodeChildrenIterator/NodeChildrenIterator/
ChildElementIterator is now peekable
s/peek().is_some()/!is_empty()/
auto merge of #3864 : brunoabinader/servo/node-peekable-childelement, r=Ms2ger

This change avoid using ```child_elements().count() > 0``` approach (using ```child_elements().peek().is_some()``` instead).