LibWeb: Implement "attempt to populate the history entry's document"

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()`

Userland/Libraries/LibWeb/DOM/Document.h

@@ -470,6 +470,8 @@ public:
 
     void make_active();
 
+    void set_salvageable(bool value) { m_salvageable = value; };
+
 protected:
     virtual JS::ThrowCompletionOr<void> initialize(JS::Realm&) override;
     virtual void visit_edges(Cell::Visitor&) override;