LibWeb: Let Document have a direct GCPtr to its containing Web::Page

With this change, Document now always has a Web::Page. This means we no
longer rely on the breakable link between Document and BrowsingContext
to find a relevant Web::Page.

Fixes #22290

Userland/Libraries/LibWeb/DOM/Document.h

@@ -572,6 +572,7 @@ private:
 
     Element* find_a_potential_indicated_element(FlyString const& fragment) const;
 
+    JS::NonnullGCPtr<Page> m_page;
     OwnPtr<CSS::StyleComputer> m_style_computer;
     JS::GCPtr<CSS::StyleSheetList> m_style_sheets;
     JS::GCPtr<Node> m_hovered_node;