Move find_fragment_node to Document.
Remove the fragment_node field from Page (fixes #3543).

Since Page isn't reflected, it cannot be rooted, and pointers to JS-managed
objects inside it are not guaranteed to be traced. This may cause the pointer
to become dangling.
Clear the fragment_name field when resizing.

This reinstates the behaviour from before the change in
a1bf49c1ce61e4df1b788960826b45c6469ad976.
auto merge of #3558 : Ms2ger/servo/fragment-node, r=jdm