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.
