LibWeb: Handle scenario when Window property setter is called on global

This commit fixes a regression introduced in
1528e9109c.

Turns out that the type of `this_value` in the property setter of the
Window object depends on how the variable is accessed. If the property
is accessed as a global variable, then this_value is of type `Window`.
For example:
```js
performance = null
```

However, when it is accessed as a property of the window object,
`this_value` is of type `WindowProxy`. For example:
```js
window.performance = null
```

This commit updates the window property setters generator to handle
both scenarios.

With this change https://discord.com/login works again.

Tests/LibWeb/Text/input/global-performance-property.html

@@ -0,0 +1,9 @@
+<script src="include.js"></script>
+<script>
+    test(() => {
+        // Test passes if this script does not throw an exception.
+        performance;
+        performance = null;
+        println("PASS");
+    });
+</script>