Do not expose `XMLHttpRequest.prototype.shouldBePrevented` in AG CoreLib app's scriptlet

[piquark6046]

Currently, a webpage can detect what a scriptlet filter in AdGuard CoreLib app ¹ tries to mock XMLHttpRequest API. AdGuard Browser Extension MV2 ² (maybe MV3 too) is not affected.

Step to reproduce:

1. Add example.org#%#//scriptlet('prevent-xhr', 'example.org/call method:OPTIONS')
2. Open https://example.org/
3. Open the web browser's dev tool and nagivate to the console tab.
4. Execute:

(() => {
  function reqListener() {
    alert(this.responseText);
  }
  const xhr = new XMLHttpRequest();
  xhr.open("OPTIONS", "https://example.org/call");
  console.log(xhr.shouldBePrevented)
  xhr.send();
})();

Related to #386

Footnotes

1. I tested with AdGuard for Windows 7.22.9. ↩

2. 5.4.3.1 on Firefox Developer Edition 153.0b6. ↩