Fix regression that no update to Page.fragment_node.
Make UIEvent.view be in a Cell.
Make Document.implementation be in a Cell.
Remove the implementation OptionalSettable for Option<JS<T>> to remove needless '&mut self'.
Remove needless lifetime parameter from LayoutNodeHelpers.
Remove needless '&mut self' from node.rs.
auto merge of #2535 : saneyuki/servo/mut, r=Ms2ger

Related #2514