Rename base_url to final_url in ScriptTask::load.

This seems like a clearer name, as it's used for more than just as a base url.
Pass a borrowed Url to parse_html.

It does not need to take ownership, and I want to reuse final_url in the
caller.
Reuse the final_url in ScriptTask::load.

In both the javascript and non-javascript url cases, final_url is set to the
same url as is stored by the Page.
auto merge of #4305 : Ms2ger/servo/script_task, r=jdm