Tracking annotations are used in a few different places:
* devtools label requests from tracking domain as such
* control center indicates the presence of trackers based on annotations
* [https://searchfox.org/mozilla-central/rev/d4b9e50875ad7e5d20f2fee6a53418315f6dfcc0/netwerk/base/nsChannelClassifier.cpp#1028-1030 lower the priority of tracker loads] in necko
* [https://searchfox.org/mozilla-central/rev/5dbfd833bbb114afe758db4d4bdbc5b13bcc33ef/netwerk/base/nsChannelClassifier.cpp#244-263 enable throttling of these requests] in necko (flag added in [https://bugzilla.mozilla.org/show_bug.cgi?id=1360580 bug 1360580])