Stop passing DOMStrings via borrowed pointer. (#1201)
auto merge of #1209 : saneyuki/servo/domstr, r=jdm

This will fix #1201.
This maybe the work in progress.

@jdm:
* Should we pass `DOMString` directly to `dom::utils::null_str_as_...`?
* Do you have any good idea to update the comment in `CodegenRust.py`?