Implement ParentNode.children for Document and Element

Also implement it for DocumentFragment
auto merge of #2192 : evilpie/servo/children, r=Ms2ger

Fixes #2186
Implement Element.localName.(fixes #2188)
auto merge of #2209 : lpy/servo/issue2188, r=Ms2ger

see #2188
Update expectations.
auto merge of #2211 : Ms2ger/servo/update-expectation, r=Ms2ger
Add NodeHelpers::get_bounding_content_box().
Add NodeHelpers::get_content_boxes().
Add Page::hit_test().
Add Page::get_nodes_under_mouse().