LibJS: Always keep a reference to the global object in Shape

We need to move towards supporting multiple global objects, which will
be a large refactoring. To keep it manageable, let's do it in steps,
starting with giving Object a way to find the GlobalObject it lives
inside by asking its Shape for it.

Libraries/LibJS/Runtime/Object.cpp

@@ -91,7 +91,7 @@ Object::Object(Object* prototype)
         m_shape = interpreter().global_object().empty_object_shape();
         set_prototype(prototype);
     } else {
-        m_shape = interpreter().heap().allocate<Shape>();
+        m_shape = interpreter().heap().allocate<Shape>(interpreter().global_object());
     }
 }