After moving to navigables, we started reusing the code that populates
session history entries with the srcdoc attribute value from iframes
in `Page::load_html()` for loading HTML.
This change addresses a crash in `determine_the_origin` which occurred
because this method expected the URL to be `about:srcdoc` if we also
provided HTML content (previously, it was the URL passed along with the
HTML content into `load_html()`).