Backport - Warning that favicon load request got cancelled is confusing

After Mozilla fixed #18513 (moved) a confusing warning is often visible in the console:

When cancelling a request for chrome://browser/skin/preferences/in-content/favicon.ico because the inner window was destroyed or a new favicon was loaded for it, it was already canceled! PlacesUIUtils.jsm:109:0

Mozilla seems to fix this with https://bugzilla.mozilla.org/show_bug.cgi?id=1279650. We could think about backporting this to Tor Browser.

This was first reported on our blog with https://blog.torproject.org/blog/tor-browser-65a2-released#comment-196614.