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/HTML/Navigable.h

@@ -11,6 +11,7 @@
 #include <LibWeb/Forward.h>
 #include <LibWeb/HTML/HistoryHandlingBehavior.h>
 #include <LibWeb/HTML/POSTResource.h>
+#include <LibWeb/HTML/SourceSnapshotParams.h>
 
 namespace Web::HTML {
 
@@ -60,6 +61,8 @@ public:
 
     Variant<Empty, Traversal, String> ongoing_navigation() const { return m_ongoing_navigation; }
 
+    WebIDL::ExceptionOr<void> populate_session_history_entry_document(JS::GCPtr<SessionHistoryEntry>, Optional<NavigationParams>, Optional<String> navigation_id, SourceSnapshotParams const&, Function<void()>);
+
     WebIDL::ExceptionOr<void> navigate(
         AK::URL const&,
         JS::NonnullGCPtr<DOM::Document> source_document,