Implements:
https://html.spec.whatwg.org/multipage/browsing-the-web.html#attempt-to-populate-the-history-entry's-document
This is going to be a replacement for `FrameLoader::load()` after
switching to navigables.
Brief description of `populate_session_history_entry_document`:
- If navigation params have url with fetch scheme then DOM document
will be populated by fetching url and parsing response. This
is going to be a replacement for `FrameLoader::load(AK::URL&)`.
- If url in navigation params is abort:srcdoc then DOM document
will be populated by parsing HTML text passed in document resource.
This is going to be a replacement for `FrameLoader::load_html()`