feat(btoa): port and modify from Array.findindex

The polyfill function argument is wrong. You don't need to reference `arrayobject`  from the argument list. Because `findIndex` should be a prototype method of Array, that means you can call it like this:
```javascript
[1, 2, 3].findIndex(function(){ });
```
And in the polyfill function, you can reference `this` to get the array instance.