fix: blocked bg-images do not trigger external content warning

[ThorstenSuckow]

No description provided.

[ThorstenSuckow]

The Performance API provides interfaces for querying resources loaded by a window-object:

var resources = document.getElementsByTagName("iframe")[0].contentWindow.performance.getEntriesByType("resource");
resources.forEach( resource => {
    console.log(resource);
});

see https://developer.mozilla.org/en-US/docs/Web/API/PerformanceEntry/entryType