LibWeb: Make HTML::Window::page() return a Page&

Author: https://github.com/awesomekling
Commit: 9793d69d4f
Pull-request: https://github.com/SerenityOS/serenity/pull/22299
Issue: https://github.com/SerenityOS/serenity/issues/22290
Reviewed-by: https://github.com/ADKaster
Reviewed-by: https://github.com/Lubrsi

Userland/Libraries/LibWeb/HTML/Plugin.cpp

@@ -56,8 +56,7 @@ Vector<String> Plugin::supported_property_names() const
 {
     // The Plugin interface supports named properties. If the user agent's PDF viewer supported is true, then they are the PDF viewer mime types. Otherwise, they are the empty list.
     auto const& window = verify_cast<HTML::Window>(HTML::relevant_global_object(*this));
-    VERIFY(window.page());
-    if (!window.page()->pdf_viewer_supported())
+    if (!window.page().pdf_viewer_supported())
         return {};
 
     // https://html.spec.whatwg.org/multipage/system-state.html#pdf-viewer-mime-types