Make some changes to javascript scheme url evaluation to bring it
closer to
https://html.spec.whatwg.org/multipage/browsing-the-web.html#navigating-across-documents

- Evaluate the js before the page load so that it happens in the
  correct `window` global.
- If the result is not a string, the response should be a 204.

This required saving some data in load_data, since it's not
possible to modify the response at the point where we're evaluating
the js.

Problem:
  After aborting on a 204 or 205 status code, you could no longer
  follow links on the page.

Cause:
  `constellation.rs` ignores new LoadUrl requests since the aborted
  one is in its `pending_changes` list.

Solution:
  Send a message to constellation that lets it clean up its
  `pending_changes` list.

- move test to correct directory
- update it to be more concise

I also confirmed that this test passes in Firefox.

- move it to its own function
- move the `url = "about:blank" code into the same block
- move the `use` statement to the top of the file

- generalize the eval_js_url function so it can be called from
  multiple places.
- call it in htmliframeelement.
- if the js eval results in a non-string result, then it won't
  navigate to a new page, so don't block on the new page loading.

Remove this block and unindent the code one level.

Doing this required cloning `load_data.url` so that we could
later mutate `load_data.url` in the same block.