Simplify the content_changed call in ScriptTask::load.
Cleanup last_loaded_url/last_url handling in ScriptTask::load.
Pass the load data to parse_html directly, rather than in an Option.
Simplify the LoadData creation in parse_html.
Move the networking code in parse_html into ScriptTask::load.

This doesn't really have anything to do with parsing HTML, and fits in better
with the code in ScriptTask::load. In particular, all changes to Page's url
now go through ScriptTask methods.
Make parse_html's base_url argument non-optional.

It turns out the case where it woul be None can't happen.
auto merge of #4268 : Ms2ger/servo/script_task, r=jdm