LibWeb: Remove uses of obsolete PlatformObject::global_object()

This API is a relic from the time when it was important for objects to
have easy access to the Window, and to know it was the global object.

We now have more spec-related concepts like relevant_global_object and
current_global_object to pull the Window out of thin air.

Userland/Libraries/LibWeb/Internals/Internals.h

@@ -46,6 +46,9 @@ private:
     virtual void initialize(JS::Realm&) override;
 
     void click(double x, double y, UIEvents::MouseButton);
+
+    HTML::Window& internals_window() const;
+    Page& internals_page() const;
 };
 
 }