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()`).
These classes are used as-is in all chromes. Move them to LibWebView so
that non-Serenity chromes don't have to awkwardly reach into its headers
and sources.
This will help a lot with developing chromes for different UI frameworks
where we can see which helper classes and processes are really using Qt
vs just using it to get at helper data.
As a bonus, remove Qt dependency from WebDriver.
